Transom Extension

OK gang,

I have read a bunch on this board and a few others about transom extensions and IOR boat remediation. I am thinking about having some glass stuck on the back of my boat. The whole thing is in very initial stages, but the drawing below shows my boat's elevation. The dark shaded area is what I am thinking the back end should look like with a more normal back end and a lower rudder.

I would very much welcome opinions.

Boat is 40' with 9000lbs ballast.

Mahalo

 

Drawing

K, here's the drawing.

 

Not sure what kind of feedback you're looking for, although real pictures, with the boat both in and out of the water would help. - Of the transom & boat's lines viewed from the stern, side profile, & various angles with her out of the water if you can manage it.

I've got plenty of both design & build ideas, but again, more info's needed. Is the hull built with cored construction back there, & if so how thick are the skins (and of what material)? Ditto on the transom. That & is there anything problematic about adding laminate, reinforcements, & cores to the inside & outside of the hull back there?
Like say a bulkhead, or structural rigging fittings real close to where new material would need to be attached.

From your picture, it shouldn't be a difficult project. All you'll be doing is extending the boat's lines, so that the transom doesn't drag in the water, especially at higher speeds.
By doing that you'll be essentially increasing the waterline length, as well as adding more surface area so that it'll be easier to get the boat to plane. Assuming that she has the horsepower to weight ratio for such.

Ah, one key thing. If you race, have a bit of a chat with the rating board. As, at a minimum, they'll likely penalize you a bit for such a mod. AKA change your rating.

 

Thanks for being a good sport Uncivilized. The hull is solid glass all over and there is nothing prohibiting addition of bulkheads etc except the rudder wires and gears are there.

I don't race seriously so having a better rated boat can only be a good thing honestly.

This whole discussion could be moot because I just spoke to a glass guy and he basically said it would be about the same cost as buying another boat.

 

An extension will not cost that much. If all you want is to streamline the waterflow, a sugar scoop is all it needs. It can be built of plywood or foam and then glassed over.

 

This isn't particularly hard and I've done similar occasionally. "Shoot the Moon" has a "bustle" intended or added. These used to be pretty common among the early IOR designs.

What you're doing is different and simply an extension. Wrap thin plywood around the stern and use this as the mold to cast the outer skin. Of course tab in the outside once you pull the mold off the hull.

 

Thanks for responses.

PAR. It is exactly that bustle that I want to eliminate, giving her a fairer line and more width of volume further down as well as a longer LWL. The glass guy gave me a ballpark quote of 45k and said it wouldn't be worth it

It may be I just need a different boat, but this one has some really good qualities.

 

Since i have 5min i will try to help. Adding an extension will alter the balance of the boat, however in this case hiding the bustle will offset that so could be ok.
Your more serious problem is that you will add a lot of volume to the back and this will need a significant aft shift to the cg.
Unless you are cutting the bustle away i would check my numbers on the longitudinal volume and cg to avoid any nasty surprises

 

Thanks Fastwave.

Yes, the idea would be to cut out the bustle and modify and drop the rudder.

 

Oh, I see what you mean! Like cutting is the way to go rather than just adding a peice over the existing hull. Thanks, that is valuable, I didnt realize the weight was such a thing.

 

I added an extension to my transom 2 years ago. It was not hard to add the extension. The most time consuming part was the preparation for painting the extension. Lost of sanding. Send me a private message and I will give more details.
Chuck

 

A transom extension is a wholly different thing than removing and fairing in a bustle. Chuck's boat looks to have lowered the centerline a bit, but other wise simply drawn the hull lines aft, to the new transom. In his case he may have added some WL length, though the balance of the yacht likely wasn't affected much.

Removing a bustle is quite different. You'll have to cut off the bustle and refair in the bottom, probably as far forward as the aft end of the fin. You'll lose some hull volume and displacement, which will have to be replaced to trimmed out. Coupling an extention with this might help, depending on what model yacht it is, where the bustle starts and stop, lose of volume, etc.

Yes, it can be done, but to what end - in other words, how to justify such serious major surgery. What year, make and model IOR is this?

 

Many years ago the designer Hugh Welbourne, who posts on here about his DSS foils, did an article in Seahorse magazine about converting 1980s IOR boats for IMS rating. He may be able to give you a quick estimate of the issues.

Another designer said to me that filling in IOR sterns rarely create the hoped-for gains, because you can't do anything about the midsection and therefore the shape remains sub optimal. In a boat like this you can also end up with a longer waterline but narrower stern. As some of the Mull designs created around the time of your boat demonstrated, a very long but skinny stern actually doesn't seem to do much for speed.

When I checked some time ago, IOR boats that had not had their hulls altered were considered more valuable in the Uk because the rating advantage of keeping the original hull more than compensated for the slight lack of speed. It may be the same with your PHRF board.

I recall reading as a kid that Imp, one of the first wide-stern medium-displacement IOR boats, was about 1% or a bit less quicker than her fine-stern contemporaries. Some of that was in her construction, not her shape. Similarly, the wider-stern Peterson designs were considered to be little if any faster upwind or downwind in the light than the similar boats of the previous "pintail" generation like yours. The wider stern boats did gain off the wind in a breeze, but while they had a significant advantage in Grand-PRix competition it wasn't as if they had afterburners on all the time.

 

Yep, that's about it, the real problem is the shape of the boat, so it conformed under the rule, which did ugly things to the entry and stern sections in particular and the midship section suffered as a result too.

A lot depends on which design it is, as each designer took various approaches to the rule package, so some can be fooled with a bit, while others are about as good as the shapes will permit. It also depends on what you want from the boat. A butchering like this will be a rate killer for sure, but if not interested in around the buoys stuff, you're not going to gain much, in terms of pointing or downwind ability. In fact, unless you are really good at yacht design, you'll probably slow her down a tad.

 

Wow. Thank you guys for being so generous with your time and wealth of knowledge. You have talked me out of the project. I always think a big part of a happy life is picking your battles; thanks for helping me not pick this one.

Mahalo!