Extending boat hull with engine mount?

[konaken]

My 26' Blackwatch sank @ the dock last year due to excessive aft weight created by switching from 200 hp 2 stroke Yamis to 225 hp Yamis. Final sinking occurred subsequent to topping off fuel with 240 gallons of gas. The twin engines were attached by a Gil mount, which afforded a marginal flotation ability. In order to prevent reoccurrance, I wish to have a local boat repair firm build a larger engine mount out of aluminum. They wish to extend the new mount all the way to the keel, thereby affording maximum amount of flotation, as well as making the boat hull longer by approximately 30". Armstrong seems to be a leader in design and construction of these. They have advised against going all the way to the keel. They have told me that they never go closer than 3-4" from the keel. I have also read elsewhere that you should be at least 2" above the bottom of the hull thus allowing the prop to work in the pressure wave behind the hull, and that if you extend the pod in line with the hull bottom the pressure wave then is behind the prop again. Can someone set me straight on this? My main intent here is to get the boat to set level at the dock and avoid repeat catastrophe of sinking. In no way do I wish to tamper with the hydrodynamics of a Bertram designed hull. Thank you.

[charmc]

Originally Posted by konaken

My 26' Blackwatch sank @ the dock last year due to excessive aft weight created by switching from 200 hp 2 stroke Yamis to 225 hp Yamis. Final sinking occurred subsequent to topping off fuel with 240 gallons of gas. The twin engines were attached by a Gil mount, which afforded a marginal flotation ability. ... Can someone set me straight on this? My main intent here is to get the boat to set level at the dock and avoid repeat catastrophe of sinking. (

There is something missing in your account. Even if you were switching from 200 hp 2 cycle to 225 hp 4 cycle Yamahas, the weight difference is less than 75 lbs/engine, or less than 150 lbs total. There is one 200 hp 2 stroke weighing about 114 lbs less, but that would be a total difference of 228 lbs; still negligible on a 26' hull. Did you add larger fuel tanks or relocate them when repowering? If so, that may have contributed more to the problem than the engine change. Were the previous engines mounted on the extension or was that added when the boat was repowered? If added with the new engines, it was the transfer of weight aft which had more effect on trim than weight per se. However, I suspect that your real problem is that some port in the hull, possibly a scupper for cockpit draining, became submerged with the trim change. That's very different from a simple overweight situation causing the boat to settle until the transom top submerged, which is what your opening sentence seems to describe. The issue of at rest trim change from the original boat design is a critical element of any repowering and/or outboard extension mount, and an important reason to be sure a local shop has the services of a qualified NA or boat designer included in their cost estimate, or that you purchase a mount from a firm with the experience and warranty of Armstrong, Porta, or their equivalents.

Once that is settled, you can consider the step vs flush bottom question. There are reasonable sounding arguments on both sides, but my personal thought is that the less hull surface in the water at speed, the less friction. Armstrong appears to believe this, also. Not conclusive, perhaps, but a good beginning.

[charmc]

The subject of hull extensions and performance was discussed recently in this thread. It might be helpful:

http://boatdesign.net/forums/showthread.php?t=19527

It appears that there are at least 2 schools of thought on the subject, as in most issues of boat performance.

[konaken]

Charlie: The boat was originally powered with the 2 strokes and the Gil mount. In hindsight, I suspect the boat always sat low in the hind end. After the sinking, I contacted Gil looking for more bouyancy, and they suggested I call Armstrong. They build their mounts conforming to the original hull design, but hold them up3-4", to avoid messing with the performance of the original hull design. I have been told if I extend the mount all the way to the keel, the boat might have a tendency to porpoise. So my thinking is to perhaps shoot for more breadth on the alteration, than go for matching the keel or hull design. I'm listening, and appreciate your input! Ken

[charmc]

Ken,

Armstrong's wider extension will give more bouyancy at rest; the extra cost may be worthwhile. I'm still concerned about trim; I'd get some expert advice to be sure all drain ports will be more than a few inches above the water at rest. Armstrong may be able to go into this in detail; they've got a lot of experience. As for performance, I'd follow Armstrong's recommendation. A full depth bottom conforming extension would be the same as buying a longer boat.

For more information on this subject, read this article by a marine surveyor. It covers the issue of scupper clearance and ways to improve things without spending a bundle. The comments on boat design were enlightening:
http://www.yachtsurvey.com/sinking.htm

[Ikenzu]

Have we figured out where the water came in yet? Aside from curiosity, it seems like a vital piece of info for your retrofit.

[juiceclark]

I restored a 1971 24' Wellcraft Airslot. Took out the twin I/Os and installed a big bracket with twin Suzuki 140s. It made the boat ride better, back down better, sit better...hell everything was just better. Too bad that design never caught on...really rode well.

I sold it a few years ago...see the boat is still on the fan webpage:

http://www.hsmarine.com/WellcraftAir...ners/TonyC.htm

Take the original hull and extend it a couple feet and get the ass outta' the water a bit...genius:

http://www.hsmarine.com/WellcraftAir...0/Brochure.jpg

[konaken]

Originally Posted by Ikenzu

Have we figured out where the water came in yet? Aside from curiosity, it seems like a vital piece of info for your retrofit.

Without question, water entered thru the scupper drains. You can see the waterline was level with the top of the rubber flappers. I intend to replace these with those golf ball looking check valve drains.