13-footer: Curved and Straight Transoms

[Bwtragic]

Q1: what was the epoch when a 13-footer's transom shape was changed to straight from curved, if indeed that change occurred?

[Moderator's note: the most reliable answer to Q1 is given much later in this discussion, with the epoch for a flat area for the engine mounting occurring in "13-foot models after 1986..." as mentioned by the Boston Whaler Owner's Manual.]

BACKSTORY
I have a two 13-footers: one is a 1979 13-footer with a curved transom; the second is said to be a 1990 13-footer with a straight transom (like my 1999 TENDER 11). I have not checked the year model of the 1990 and the stencil number is long gone. And, yes, I have three Boston Whaler boats currently.

I have not found any mention on continuousWave.com of the epoch of when the change in the transom shape of the 13-footer occurred, other than the 13 GLS model has the straight transom.

Both 13-footer have the same engine splash well--by looks but I haven’t measured them to confirm.

On the 1990 13-footer the transom is pretty rough. and I am not sure if it is original or has been rebuilt. I am hesitant to start digging with an angle grinder to work out what is right with it.

I hope to deduce if my 1990 13-footer originally had a curved transom that was later re-done as a straight transom; the skin either side of the ply is way too rough to be original.

The 1990 transom is now sound, but fair is certainly not a word you’d use to describe it.

[jimh]

Looking at a Boston Whaler original drawing for the 1997 GLS 13

https://continuouswave.com/whaler/refer ... ersary.pdf

I do not see that the shape of the transom is "straight", but at the engine mounting location it does appear to have a flat section.

If you compare with earlier drawings, such as one from 1971

https://continuouswave.com/whaler/refer ... s-wood.pdf

or

https://continuouswave.com/whaler/refer ... 13Wood.gif

The transom is not completely flat in the engine mounting area.

In the 1977 catalogue there is a color photograph that shows the transom and engine mounting area of a SPORT 13. The engine mounting area appears to be flat.

Fig. 1. A photograph from the 1977 catalogue shows the transom of a SPORT 13. The engine mounting area appears to be flat.

1977_13.jpeg (85.5 KiB) Viewed 758 times

In the 1996 catalogue there was a line drawing of the 13 STANDARD; it shows a curved transom with a flat area for the engine. The original scan is rather low-resolution. Here is an excerpt from that drawing that shows the 13-foot hull in c.1996:

Fig. 2. A drawing from an illustration in the 1996 catalogue that shows the 13 STANDARD hull and its transom shape. Clearly the engine mounting area is flat.

lineDrawing1996_13-hull.jpg (19.46 KiB) Viewed 759 times

My inference: when the transom design for an engine with 20-inch-shaft occurred in c.1973, the section of the transom where the engine would mount became flat. The photographs in the catalogs do not reveal this because they just are not shown in the proper view.

[Bwtragic]

Thanks Jim, as per [Figure 2] that is what I refer to as a flat transom.

[Moderator's note: the purpose of having illustrations identified by a figure number is to permit readers to refer to the illustrations in an unambiguous manner by mentioning the figure number when necessary to refer to them.]

The [newer of my two 13-footers] has a production date of October 1990. The [older of my two 13-footers] has a black stencilled number in the engine [splash well] beginning 2C and a tag identifying it as April 1979.

The 13-footer from April 1979 has the curved transom and must also be a 20-inch transom, ex-factory I believe. The curve is consistent gunwale to gunwale, I daresay you are familiar with it.

[My inference is] perhaps the change [to something unsaid] occurred later in the model cycle than 1973.

That such a significant visual change appears undocumented in the model reference section is curious.

[jimh]

The 13-footer from April 1979 has the curved transom and must also be a 20-inch transom, ex-factory I believe.

I do not understand your intended meaning of "ex-factory."

The 13-foot hull by 1979 would have had a transom designed for a 20-inch-shaft engine.

[My inference is] perhaps the change [to something unsaid] occurred later in the model cycle than 1973.

Your remark is not clear to me. What feature of the 13-footer specifically are you suggesting changed and how exactly how much "later in the the model cycle than 1973" do you believe this feature changed? If you specified the feature and the date, your opinion would be more clearly stated.

[jimh]

Regarding the early 13-foot hulls having a curved surface in the area where the outboard engine would be mounted:

In the 1960's the typical engine mounting arrangement for a smaller horsepower engine that might be used on a 13-footer would be as follows:

• the engine would be set on the transom, resting on the top of the transom;
• the engine would be fastened to the transom by two circular pads that had a form of universal joint at their point of attachment to the threaded section of the mounting bolt, so they could adapt to any particular angle offset of the transom from precisely perpendicular to the mount-tightening bolt;
• the mounting bolts would have extension handles to allow hand tightening of the mounting bolts against the transom;
• the mounting bracket itself did not have a large flat plate section, but was typically two vertical relatively thin sections that would bear against the transom.

An engine mounting arrangement like this would tolerate a slight curve in the transom.

See the illustration below:


Fig. 3. A very early 13-foot hull with a 25-HP Johnson engine whose mounting arrangement was typical of that epoch.

It was also often seen that the two handles on the mounting bolts would be position so they were horizontal and pointing toward each other. There were holes in the handles at the top. A pad lock could be fitted into the two holes and closed, locking the handles in place as a means of preventing unauthorized removal or accidental loosening.

[jimh]

That such a significant visual change appears undocumented in the model reference section is curious.

The REFERENCE article has been on-line for more than 25-years, and your citing of an omission of mention of the curved transom in that article is the first comment of that nature about that article.

The topic of the curved transom has been discussed before on continuousWave. Here are a few references:

Cetacea Page 56
https://continuouswave.com/whaler/cetac ... age56.html
Read the caption for Figure 56-03

Mounting an Evinrude 40 to a 1962 13' Whaler
https://continuouswave.com/ubb/Forum3/HTML/020370.html

1963 13-footer Curved Transom
https://continuouswave.com/ubb/Forum3/HTML/021045.html

A 20-inch-shaft Engine on a Classic 13
https://continuouswave.com/forum/viewtopic.php?t=64

In addition to the curved nature of the transom, a further problem was that on some engines the engine clamp mounting bracket did not bear flat onto the transom due the transom's top being at a slight angle. Shims were required on some engine brands (notably on Mercury engines) to allow the weight of the engine and its mounting bracket to become flush and level with the top of the transom. This is mentioned in the FAQ at

FAQ: Q8: How does the engine mount ot the transom?
https://continuouswave.com/whaler/reference/FAQ/#Q8

The reason that OMC engines did not need the shim was probably because the transom of the early 13-footer was designed specifically to fit the OMC mounting bracket used at that time.

[Bwtragic]

Again, thank you Jim. Not after starting ww3 here.

To reiterate I have a 1979 model that has a curved 20” transom and I have a 1990 model that has a straight transom. And to be clear the curved transom version looks like it came from the factory as 20”.

The reference section details a change to the transom height as of 1973 so it’s reasonable to assume that the 1979 with a curved transom was manufactured as 20”.

My 1979 hull therefore proves that the change to the straight transom did not occur in 1973.

I know you like your facts to be correct but in your second post in this thread you finished by concluding/inferring the transom change to a straight pad was in 1973.

Again, this was a not insignificant change to the hull to go from a curved engine mount to a straight engine mount transom, perhaps the reference section lacks this detail and perhaps our collective audience might be able to define when the change occurred.

[jimh]

I think there are three separate elements that are being discussed:

• First, is the 13-footer transom slightly curved;
• second, even if the overall transom is curved slightly, is the engine mounting area of the transom a completely flat plane surface, and
• what was the proper engine shaft length for mounting an engine on the transom.

My view is that the transom of the 13-footer

1. was originally curved and remained sightly curved; but perhaps at some point became completely flat across the entire stern of the boat, with the epoch to be discovered;
2. at some point the center section of the transom was changed from a slight curve to include a center areas with a completely flat plane for mounting the engine, with the epoch to be discovered; and finally
3. there was a change from a notched transom intended for engines with a 15-inch-shaft length to an un-notched transom intended for engines with a 20-inch-shaft length, and this occurred in c.1972.

The 1996 Catalogue drawing (that is shown above as Figure 2) should be considered reliable evidence that by 1996 the change to having a flat engine mounting area had occurred. But the period between the change to a 20-inch-shaft height transom (which seems to be 1973 to both of us) and when the transom engine mounting area became flat is still unclear.

I suggested that a catalogue photograph from 1977 appeared to show a flat transom, but your first-hand account that you have a 1979 13-footer with a 20-inch-shaft-height transom and it is curved (and thus the must be interpreted as the engine mounting area is not flat) seems to disqualify my interpretation of the photo.

What is needed is a better date benchmark for when the transom engine mounting area became a flat surface, even if part of an overall slight curve to the transom.

What is your best estimate?

What evidence to you have to support it?

Also now still not clear to me:

When, if ever, did the 13-footer transom change to become an entirely flat transom from a transom with a slight overall curve?

When all these dates can be established with some good evidence, I will be very pleased to make changes to the REFERENCE article to reflect the stages of change to the transom of the 13-footer.

[Bwtragic]

To clear up something, both transoms are curved either side of the engine, the 1979 carries the curve through the engine mounting area whereas the 1990 has a flat engine mounting pad identical to the 1996 drawing. I have not seen a genuine 13’ that has a completely straight transom but any Boston Whaler is a rare thing in Australia.

Trivially the 1979 has a 40-HP tw0-cylinder Mercury USA two-stroke-power-cycle engine and is slightly faster than the 1990 with a Yamaha 40-HP engine. The 1979 certainly trims better but the 1990 has its weight set further back as well as the heavier engine.

Visually I prefer the 1979 transom. It is quirky and distinctive in a way that only a Boston Whaler can be.

[jimh]

Perhaps the final word on the curvature of the transom on the 13-foot hulls comes from Boston Whaler themselves, where they mention in the Owner's Manual in the Operation section in the Engine Installation section the following:

On 13-foot models (prior to 1986) the transom is slightly curved. If on larger, newer engines there is a space between the engine and the transom, washers can be used to fill the space to prevent bending or placing too much pressure on the engine bracket when the bolts are tightened up. 13-foot models after 1986 have a flat raised area on the transom for engine mounting.