Engine Brackets

This article describes the improvements gained by mounting outboard engines on setback brackets and how such brackets were offered from Boston Whaler in the period of 1983¹ to 1993². (A companion article shows in detail the fitting of an aftermarket setback bracket to a Whaler with standard transom and the advantages this has produced, as well as details about the use of hydraulic steering, an upgrade frequently required when engines are mounted on brackets.)

Why Brackets?

It is often desirable to move the mounting point of outboard engines aft from the hull's original transom by as much as 36 inches. The two principal benefits of this are:

• The propeller runs in cleaner (i.e., less-turbulent or less aerated) water that exists behind the boat.
• The engine can be raised (sometimes several inches) which reduces the hydrodynamic drag by having less of the engine lower unit submerged in the water.

The net effect of these changes is to increase the propeller efficiency (from operating in cleaner water) and to decrease drag (from reduction of submerged lower unit). This increases the potential top speed of the boat, and improves the overall fuel economy.

In addition, there are some more subtle performance benefits to this mounting technique:

• Bow lift (in static trim) may be gained from the changed weight distribution aft. When underway this in turn may allow the engine trim to become more vertical, resulting in the propeller thrust becoming more aligned with the forward motion of the boat.
• The engine thrust will be applied with a longer moment arm to the boat's center of lateral resistance and the boat's center of gravity, producing greater leverage of the engine thrust on both the boat's course and the boat's trim.
• Operation in reverse will be improved because of the increased distance between the propeller and the hull.
• Radically reduced immersion of the propeller may permit use of more efficient propeller designs, further increasing efficiency.
• Underwater drag of the lower unit may be reduced when its trim becomes more nearly vertical because of improved flow of water around the "bullet" of the gearcase.

Some of these performance gains are very slight and are really of concern only to the competitive racer, but taken en toto there is a definite gain to be had from mounting the engine behind the boat. Side by side comparison of boats with and without bracket mounted outboard engines often shows marked improvement in performance and fuel economy for the bracket model.

An additional benefit of bracketed engine mounting is it leaves the boat transom intact. There is no cutdown of the transom for engine mounting, nor are the gunwales or aft deck cut away to form a well. The result is the strength of the hull is increased. The full transom will also keep following seas out of the boat and it will prevent shipping water aboard when rapidly reversing, as when backing down on a fish.

While it might at first seem strange to mount the engine outside of the boat, the performance reasons detailed above make a case for doing so. When the engine is moved behind the boat, it releases valuable space inside the boat to other uses. The boat's cockpit becomes bigger and more useable.

On top of all these sound reasons, there is the "cool" factor. Boats with engines mounted on setback brackets just look cool. This is probably because many full-transom, engine bracketed boats are premium designs, and the appearance of a bracket mounted engine is thus associated with high-quality, high-performance boats.

What Price Bracket?

The performance and space gains from a bracket mounted engine do come with a price. First of all there is the cost of the bracket, which can approach $4,000 for larger, more exotic designs. It is also usually necessary to switch to hydraulic steering, and this is also expensive, typically running close to $1,000 additional cost compared with cable steering devices. The brackets themselves add weight, always a negative for performance, and they also create problems when trailering. The load of the engine weight is cantilevered off the transom, producing especially acute bending forces on the transom when the boat is bouncing down the highway on a trailer. With long setbacks, it may not be feasible to support the lower unit(s) of the engine(s) with an auxillary load bar, as is often done when trailering an engine in the tilted-up position. Stress on the transom from the large unsupported engine weight can be considerable.

Design of bracket installations can also be difficult. Many factors must be considered. For example, if the engine(s) mounted on the bracket have fairly tall cowlings and the boat has a full transom, careful planning must be made for the amount of setback so that it creates enough room for the engine to tilt forward. With most V-6 engines, about 30 inches of setback will be needed. As a general rule of thumb in designing setbacks, one inch of setback per foot of hull is considered a good starting point. This would limit use of the 30-inch setback bracket to only the larger outboard boats, say 25-feet or longer. The bracket design must also have enough recess to permit the engine(s) and their steering tie bars and actuators to tilt downward as the engine trim is raised to full up.

The addition of the bracket's weight, the movement of the engine weight aft, and the additional buoyancy of the bracket (if any) will all combine to affect the static trim. The outcome is not guaranteed to always be an improvement. Another concern is the tendency of the boat's stern to settle when coming off plane. This may be accentuated by the lever arm effect of the bracket, possibly lowering the engine power head quite close to the water when a long bracket is used. Finally, to take advantage of shallower running it may be necessary to switch to more aggressive propellers with better holding. Standard "elephant ear" props may not give good performance when raised higher than normally operated. Thus new propellers may be needed, which add an additional cost to the installation.

The trend toward mounting engines in this manner began to have a serious following among recreational boaters in the early 1980's. In particular, the notion of raising the engine gained wide acceptance among competitive Bass fishermen, who doubly benefited from the increased top-end speed and the reduced operating draft. The trend toward long setbacks was adopted by offshore fishing boats, where rapid backing down without shipping water over a cut transom was much appreciated, as were the many performance improvements. Boston Whaler made several options available in their many hull models that permitted customers to take advantage of engines mounted on setback brackets between 1983 and 1993.

Whaler Factory Options

As supplied by the factory, Boston Whaler boats were available with several options for mounting the engine behind the original transom. Not every choice was available on every model or every year, but the range included:

1. Whaler Drive: The Whaler Drive molded appendage was constructed with the same foam-filled technique as the rest of the hull, and it added considerable reserve floatation to the boat. It was mechanically fastened to the transom with approximately a dozen bolts, augmented by six very large, long bolts that extended from the extreme aft face of the bracket, through the Whaler Drive, through the transom, and anchored inside the hull with metal backing plates. The joint between hull and Whaler Drive was sealed with caulk. Needless to say, the drive appendage was very securely fastened to the hull. The Whaler Drive extended the approximate keel and chine lines of the hull aft about 30-inches. The lower portions of the appendage ran in the water at speed, effectively extending the length of the hull and thereby adding drag and negating the less turbulent water advantage. Because it displaced water, it also added buoyancy (as distinct from floatation) to the stern of the hull, in effect compensating for its own weight and for the shift in weight of the engines farther aft. The static trim of the hull was therefore not changed much at all. Conventional outboard engine(s) could be mounted to it, locating the propeller about 30-inches aft of the boat. The Whaler Drive was a rather expensive option, and in some cases the cost might have led buyers to just get a larger boat with the standard transom. This might explain why the Whaler Drive is most often found on the 25-foot hull, the longest hull that can still be easily trailered. (The next largest model, the 27-foot could not be easily trailered due to its larger beam,)
2. Gil Bracket: The first Gil Brackets were metal engine setback brackets fabricated using welded tubing and plates. These were made by the Gil Manufacturing Co. and offered pre-installed by Whaler for a short time. Conventional outboards could be mounted on the bracket extension. The original design later evolved into an aluminum welded truss. (See below.)
3. Sea Drive: a trade name for OMC outboard engines with their own integral setback mounting bracket and power tilt, again pre-installed by the factory.^[3] This option was not too expensive (roughly $1,000) for the boat transom modifications, but the Sea Drive engine was quite a bit more expensive than a conventional outboard. This was probably the most common option ordered that resulted in delivery of a boat with a full transom.
4. Sea Drive Blank: a boat prepared for Sea Drive installation, i.e., having a full transom built up, but without the factory supplied Sea Drive. This rare option was intended to permit the owner to attach a bracket and outboard engine of his own choosing.

Whaler Drive Boston Whaler offered this expensive engine bracket appendage as an option on many models. PhotoCredit: JWH

Gill Bracket The Gill Manufacturing Company made engine mounting brackets with this tubular style of fabrication, seen here on a non-Whaler hull. PhotoCredit: unknown

OMC SeaDrive An alternative to standard outboards, OMC offered this integrated mounting bracket, tilt mechanism, and outboard motor as the "SeaDrive." Here a pair of them are mounted to the transom of a non-Whaler hull. The Florida sun has put some age on these engines. PhotoCredit: JWH

SeaDrive Blank A full transom Whaler could also be ordered and then equipped with an aftermarket bracket, like the nice installation here of an Armstrong bracket with platform supporting twin Mercury 200-HP EFI outboards on a 25-Outrage. PhotoCredit: JWH

Mounting Bolt Detail To support the weight of the Armstrong bracket shown above, at least a dozen large thru-bolted fasteners are used. On the inside of the transom they are fitted with large metal backing plates. The full transom and bracket approach also hides from view most of the electrical, hydraulic, and fuel lines that normally are seen in the engine well. PhotoCredit: JWH

Molded-On Brackets

All of the bracket designs and adaptations seen so far have been based on starting with the original cut-transom molded hull, reworking the transom into a full-transom style, then adding an engine bracket appendage, either at the factory or aftermarket. The performance gains from use of bracketed outboards have caused some manufacturers to continue to offer full transom boats for use with aftermarket brackets from Armstrong, Stainless, and others.

Some makers have adopted a different approach: they actually mold the bracket as an integral part of the hull. Former Whaler designer Bob Dougherty recently introduced the Edgewater 260, a hull with classic Whaler lines but featuring a molded-in engine bracket. This design was originated as a bracket mounted outboard, but instead of mechanical fastening the bracket appendage to the molded hull, the entire engine mounting bracket is just molded as part of the hull.

Molded Bracket This Edgewater 260 has the engine bracket incorporated in the hull molding. At planning speeds the bracket will run out of the water; at rest it will displace some water, providing additional buoyancy. This well-engineered transom has pockets molded in to permit attachment of trim tabs. Note the placement of the SONAR transducer on the keel center line. The hull shear drops just slightly at the stern, suggestive of the "Euro" look now popular, but really just a gradual transition to the final transom height at the bracket. The strength of the bracket is evident from the location of the stern lifting eyes; they are mounted to the bracket and support the majority of the weight in this lift arrangement. PhotoCredit: JWH

Molded Fiberglass Bracket

After it stopped producing the Whaler Drive option, Whaler did continue to offer a bracketed option on their 27-foot hull. This used another bracket, also constructed of molded fiberglass, but manufactured by Salt Shaker Marine, a builder of semi-custom high performance outboard fishing boats in the 30-foot range.

Salt Shaker Marine Bracket This Whaler 27 has a fiberglass bracket from Salt Shaker Marine. Whaler used these on some OEM applications with the 27-foot hull. PhotoCredit: LHG - Scan: JWH

Full Transom Hull Fabrication

For each of the four original Whaler bracket options, the hull was modified to the "full transom" configuration. This was accomplished after the hull had already been molded with the standard "cut" transom. The full transom was built up by adding additional reinforcement consisting of plywood sheets built up to thickness and doweled into the existing transom. Several layers of fiberglass were laminated on the wooden core, then this area was re-gel coated to conceal the re-work from the exterior. From outside, the transom build up is undetectable. The modification can be clearly seen from the interior, where the laminates and gel coat are not as well finished. A new molded deck section was laminated on top, capping the transom, covering the original engine well area, and forming a stern cockpit bulkhead which then created a lazarette (stern storage locker), accessible through hatch openings in the new deck.

The original drains of the engine well were plugged by the transom rework. To permit the cockpit to retain some self-bailing capabilities, large scuppers were cut through the old front bulkhead of the engine well, draining the cockpit into the old motor well (if the boat had the typical full-width splash well). If the boat had the smaller splash well, water could drain to the stern without further modification. The motor well in turn was then drained overboard by larger drains located about 4 inches higher than the original motor well drains. These sockpit drains or scuppers are finished flush with the original hull transom and are hard to see from the exterior. The original engine splash well still exists, but there are no drains for it to empty. This problem was handled in two ways. On some boats the original splash well was filled with foam, sealed, and capped. In later years the splash well may have been left open and a pump installed to expel and water that accumulated there. They change may have been made after complaints about water saturation of the foam in the filled and sealed splash well.

At first it seems odd that the full transom option would be accomplished in this manner, but from the boat builder's point of view, there is not much else that could be done, short of producing a separate set of molds for creating the full transom model boat. Creating the full transom in the mold would require new tooling for both the hull and the liner molds, a great expense which would not be economical given the anticipated mix of orders for full and standard transom model. Converting to the full transom required considerable additional labor, and craftsmen more skilled than the typical production boat molded laminate worker were needed to do this work.

Full Transom Option The standard hull with motor well was reworked into the full transom model by building up the transom and adding this molded overlay to form a deck and rear cockpit bulkhead. The enclosed space was accessed by three new hatches. The center hatch and locker area contained a molded lift-out well with drain. PhotoCredit: LHG

Based on the factory ratings for maximum horsepower, the strength of the transom on a Whaler with a factory reworked full transom should be considered greater than the strength of the standard transom model. In the case of the 25-Outrage, the standard transom hull is rated at 300-HP, while the full transom/Whaler Drive configuration is rated at 450-HP. Experience has shown, however, that standard transom boats can be fitted with setback brackets or style-truss brackets with good success with engines up to the ratings of the standard transom.^[4] Adding a small setback bracket is a not a huge modification to a boat; adding a long truss-style setback bracket is a rather significant change to the design of the boat and should be considered somewhat experimental.

Aftermarket Options for Standard Transom

If your Boston Whaler boat does not have a full transom--and most do not--you can still take advantage of the setback mounting of your engine. There are several bracket designs which can be retro-fitted onto boats with the standard transom configuration:

1. Setback Bracket: a metal bracket which provides a fixed amount (generally in the range of 6-12 inches) of setback. It also often provides for some vertical adjustment by mechanical means, usually a large threaded jack screw. These brackets usually mount to the transom in the original engine mounting holes. A number of manufacturers make brackets of this type in various designs and styles. One factor in choosing a bracket is how well it matches the size, weight, and horsepower of your engine. Brackets intended to handle 300-HP engines will look decidedly out of place on a 13-foot boat. This type of bracket generally costs under $300. An example of this and more details of the installation are shown below.
2. Jack Plate: a setback bracket which provides a minimal setback and is principally intended to provide vertical adjustment of the engine height, usually accomplished with hydraulic cylinders and capable of being remotely actuated and adjusted while underway and at speed. Brackets like this cost as much as $600 or more. Cetacea Page 41 has an example of this type of jack plate.
3. Truss-style Bracket: a large, hollow, welded aluminum truss bracket for achieving setbacks of 24-inches or more. An example of application of a long, truss style setback bracket to a Whaler with a standard, cut transom can be seen in Cetacea Page 45 on this web site. Brackets made by Armstrong, Stainless Marine, or Gil are examples of this type. Available swim platform extension can make these designs similar in appearance to the Whaler Drive, but the bracket typically does not extend the hull form and runs out of the water when underway. It may, however, displace some water at rest and thus add some buoyancy to counteract the shift in weight aft. Also, the bracket only extends the mounting aft and does not provide adjustments for height, except by the conventional adjustment provided by the engine mounting hole choices. The encapsulated air space of the hollow bracket cavity adds floatation. Mounting a long setback truss style bracket on a Whaler with a standard transom requires solving several problems. The bracket may cover the existing engine mounting holes and drains. These holes would then have to be filled and new drains cut into the transom. Standard catalogue brackets may extend too far above and below the Whaler's cut transom to utilize the bracket's normal mounting bolt locations. The upper bolts may be above or too near the top of the cut transom; the lower bolts may be below the bottom of the transom well. Consult the maker for suitability for use on your Whaler hull. Because of their longer extension, truss style brackets are generally only suitable for boats 20-feet or longer. Brackets of this style usually cost $1,500 - $2,500 and may require custom fabrication.

Trim Angle Offsets

On many of these engine mounting options there may be a fixed offset in the angle the new mount makes with the existing transom, or the mount may offer a range of adjustment of the angle. This is often desirable because some engines (notably early OMC outboards) did not have enough range in their down or lowest trim position to reach an optimum setting. This fact alone may warrant the use of a setback bracket. In some cases, an increase in down trim may be appropriate as the weight distribution of the bracket may create some bow rise in the boat's static trim.

Fixed Height Offsets

In many cases, by choosing which holes are used, it is possible to mount an aftermarket bracket in such a way that the engine mounting is raised several inches. This may permit use of a longer shaft engine than called for by the original transom design. With older Whalers that need a 15-inch transom, this may allow use of engines with the more common 20-inch lower units. Outboards with 25-inch shafts may also be mounted instead of more conventional 20-inch shaft engines. This raises the power head farther above the water and reduces the chances of the engine being swamped.

More Reading

For an excellent example of an engine setback bracket fitted to a Boston Whaler standard transom, please see my accompanying article, Cut Transoms and Brackets.

Acknowledgement

This article owes much to the careful review and input from Larry Goltz, who accomplished the expert installation of the setback engine bracket on his classic Boston Whaler 25-Outrage seen here. I have had the opportunity to spend many hours driving that boat, and the performance is outstanding.JWH

Comments or Questions

If you have a comment or question about this article, please post it to this message thread in the Whaler Forum.

Further Reading

The installation of engine set-back brackets on Boston Whaler 17-foot Montauk hulls is well illustrated by several photographs recently featured in our CETACEA collection. And even smaller 13-15 foot Whalers can benefit from engine setbacks, as seen in The Spruce Creek Navy. Mounting of engine setback brackets to boats with a cut down transom was illustrated in Cetacea Page 45.

For more information from manufacturers of brackets and others, consult these sources:

• Armstrong Marine
• B Bracket
• Stainless Marine
• SeaMount
• DIY Marine's description of the installation process of a large truss style bracket
• Article on brackets oriented toward performance boats
• Porta Marine's exotic hydraulic bracket
• RITE-HITE 6-, 8-, and 10-inch setback brackets

Notes

1. The Sea Drive option was first offered in 1983.
2. The Whaler Drive was last offered in 1993.
3. The Sea Drive was an excellent idea. There are a few existing Sea Drive engines still running in good condition, but parts are generally unavailable. Many Whalers that originally had Sea Drives have been converted to other styles of setback brackets, especially aftermarket hollow truss style brackets.
4. If your Whaler hull is still covered under its 10-year warranty you might want to contact your dealer for advice regarding brackets and ratings. In the 18-Outrage shown, the boat seems to have suffered no ill effects with engines rated at 150% of the maximum, but its outboards are generally operated well below wide-open throttle most of the time.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 1999, 2000 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified:
Author: James W. Hebert with much input from Larry Goltz.
This article first appeared February 16, 2001, and was revised November 27, 2001 as part of the original article on Engine Brackets.
Eight new photographs were added showing various brackets May 8, 2001.
The article revised and the portion showing the installation of a bracket was incorporated into a separate and expanded article on May 9, 2002.