Inboard to Outboard Conversion

Hello again everyone, I’ve got another potential project. This one may be a little more involved than the last one you can see here: http://www.boatdesign.net/forums/fiberglass-composite-boat-building/overhead-epoxy-work-45781.html


Short story: Considering converting from twin inboards to twin outboards.
Long story: Upon removal of the inboard engines last fall I discovered that there is water damage/rot in the stringer system, fuel tank bunks and transom. Fiberglass is intact, but drilling test holes reveals extensive damage. Stringers, transom and fuel tank bunks all need replaced. Why not beef up the transom, run the stringers through the transom to create a hull extension on which to mount two 225-300 hp four stroke outboards? Removed engines are sold in preparation for new power.

Why Outboards? Service much better locally than inboards. Maintenance is much easier, no winterizing. No metal in the water when tilted up. Less thru hull penetrations. Cost is only incrementally more than new gas inboards, on par with new diesel inboards. No explosion problems as with gas inboards.

In roughly this order:
Stringer Plan: Remove 4 engine stringers by cutting off the glass tops, or the top and one side where needed. Install epoxy coated 4”X8”X10’ beams in the same places with a nice epoxy bog. Completely glass over with triaxal fabric and epoxy. 30” of each stringer will extend through the current transom.

Prop Pockets: Boat currently has small prop pockets for the existing inboard props. These would have to be cut out and replaced with “smooth bottom” I’m thinking cut out, screw melamine to the underside of the hull as a form, and replace with equal thickness of triax and epoxy, beveled back into the existing hull on all sides and connect to the reinforce transom.

Transom Plan: Remove existing wet/rotted wood, go back with 2-3” of layered, staggered marine plywood coated in epoxy. Triax between each layer, tabbed back into the hull. ¾” plywood knee braces from new stringers to transom glassed in place.

Hull Extension Plan: Approx. 66” wide by 30” long by 30” high hull extension to attach outboards to. Use extended stringers to create a form for essentially outboard bracket. I’m not sure if I should build a mold, or build the bracket out of marine ply and glass in place. Glass ¾ ply stiffeners on top of the stringers to the transom and to the outboard mount. After than glass in a top cap and new swim step to the transom the full width.

COG Calculations: Removal of the inboards, installation of outboards appears to move the COG back about 0.3 feet, based on rough calculations.

I have attached some pics, how many places have I gone wrong here?

 

If you go along with the plan, remember that the risk of explosion still exists since the fuel tanks will remain inside the boat. The prop tunnels can be easily filled with foam and then skinned over with glass. Considering the shape of the hull and the huge bilge keels, you are looking at a modest speed target, so the change will not be so radical.

 

One problem with high hp outboards is they are not really suited to sedate cruise speeds, they work best on boats that cruise at 30+ knots, the gear ratios and prop selection are tailored for speed. You can't get a higher reduction and bigger wheel in there to suit 20 knot cruise.

 

16-18 knots might be more realistic for that boat, I wonder at what stage the bilge keels were added, and what good they might have done, they certainly are not compatible with going fast !

 

The Hi Thrust Yamahas and the Mercury Big Foots (Feet?) have a pretty good push for their rated HP and are not designed for high speed planing boats.

 

What HP are you talking ? I haven't seen them in main-range higher hp engines, but there may be "commercial" engines with better characteristics for this purpose.

 

This may surprise some folks but this vessel performs as a planing hull. Originally outfitted with twin 160 hp diesel engines she put in a top speed of 21 mph after bilge keels. The addition of the bilge keels did not reduce top speed, in fact may have increased top speed slightly although I also had fresh bottom paint on a well. A sister ship with the same hull and same bilge keels with twin 230hp yanmars has a top speed of 25 knots. Its well documented, see the link here: https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxsaGJvYXRpbmZvfGd4OjM5MDAwYTU0NzNiNmU1MDQ

and here https://sites.google.com/site/lhboatinfo/home

I have heard but not seen claims of 30+ mph with vortec 350s. I think the boat would respond well to more horsepower.

Bilge keels were a great addition, certainly improved handling in following seas and troughs as well as rolling at anchor.

I own a 19' aluminum river sled with a 50hp yamaha that does quite well at 20kt cruise. I also drive a 26 foot aluminum pilot house walk around with twin yamaha 115s, she cruises at 20 knots at 4500 pm, over 1700 hrs on those motors and counting so I am not too worried.

Looking at Suzukis with a the lower reduction gear would allow swinging a bigger prop.

 

I don't think anyone doubted it was performing as a planing hull but 21 mph is still a modest top speed. The Suzuki's gear ratio seems more suitable, but still not where you'd want it, and of course you have to run backed off to around 75% of peak rpm, or the fuel bill will kill you. Your boat will be running about half the speed of the sports boats these big outboards are intended for. I'd expect 1 mpg or thereabouts.

 

What would be the best way to fill the prop tunnels with foam? Build a mold around the underside and fill with pourable foam from the top?

 

I would make the transom knees out of 6 layers of 3/4" ply for a total of thickness of 4 1/2" and use 3 of them attached to the new stringers and transom, one centered and 1 centered for each outboard as they are high HP. Use 6-1/2" carriage bolts 3 in to transom 3 into stringers.

 

I think that pouring foam is a good idea. You can screw the plywood directly to the bottom with plastic over to keep it from sticking.

 

3" seems to be the maximum thickness I could use for the transom, barring a jack plate. What would be the best composite construction? 2" coosa with 1/2" epoxy triax on each side, or some combination of marine ply built up?

I've never used pourable foam, I am guessing I should expect quite a bit of expansion, would a relief hole at the top be sufficient or will it blow my mold off the bottom of the pocket?

1 mpg seems like a good estimate for mileage given what I've seen and read of these hulls with gas power.

Thanks!

 

The hole where the shaft comes out will provide more than enough relief

 

Excellent thank you, any thoughts on proper density? My research indicates 8 or 16 pound might be the best choice.

 

How about a Caudwell diesel outboard/outdrive unit, you will need to adapt the transom to fit the drive.

www.youtube.com/watch?v=d2HZmqSE4wE‎

I think they might be expensive!!!!