Need serious advice from expert builders/designers..

Hey folks - I rarely post on this site, but I do browse from time to time. I need some serious advice on my project. I'm leaning towards converting my hull to an outboard - it was a straight inboard with a keel.

1978 pacemaker - was a flybridge - being converted to open CC

starting on page 11 is the flybridge tear down, if you care to see some more pics, http://www.thehulltruth.com/boating-forum/387293-pacemaker-26-restoration-11.html


My question is - will the keel present an issue if i'm converting to a single big outboard (250+) on a bracket?

I can provide better pics of the keel if need be!

 

The issue that concerns you is what ? Cavitation in turns ? Loss of efficiency ? I think it more than likely you won't have much of a problem unless it is very close to the engine, say less than 4 feet. Might be more trouble if twins.

 

Mr Effiecieny, thanks for the response - i guess all of the above is what could be of concern. I do think it will pick up some top end, and easier maneuverability,ect with outboard on a bracket.. If I decide to go that route, would removing the keel be worth while? Or do you think the keel wouldn't affect performance negatively enough to warrant removing it..

thanks for the insight

 

Provided that the outboard prop receives clear water, the keel will not be your issue. Better pictures indeed may help here: I could not access your link without subscription.
You should give a thought of longitudinal trim after alteration.
New weights and their positions may not (yet) match with the hull shape.
The transom needs to be checked for strength and most likely needs to be reinforced.

 

Yes, the transom will be reinforced - multiple layers of penske board and lots of glass

Weird , that link should work - I'm not sure why it's asking for a log in as it is a public forum...

If I were to guess, the keel ends about 2-3 feet from the transom. I would probably go with a 30" bracket to get the motor furthest back from the keel in clear water

 

The boat has not a great deal of deadrise aft by the look of it, so maybe it needs a keel to keep things on track. Sure it represents additional drag if in fact the boat would be tracking well enough without it, but once you cut it away, going back won't be easy. And there may be structural ramifications to it as well. And as has been said by HakimKlunker, you can't just bolt a pod on and off you go, the transom as is was not designed for those stresses, having been an inboard.

 

Besides the transom, the stringers may need to be modified. An inboard places the thrust somewhere close to the center of the hull, while and outboard places the thrust on the transom.

 

The keel as you call it is a skeg and unnecessary on an outboard or out drive equipped boat. It's removal will provide a cleaner flow to the prop, which is desirable.

The bracket should be tied to the engine bed stringers on the inboard face of the transom (big knees, preferably metal), but no further modifications will be necessary. This will allow the bracket loads to transmit to the hull shell bottom, which is where they need to be.

I agree the transom might benefit from some additional reinforcement, which amounts to making it thicker and generous tabbing to the hull shell. The transom should be 2.75" (70 mm) to 3" (76 mm) thick for this amount of HP on a bracket. 2.25" (57 mm) if transom mounted.

A bracket for this boat should be well engineered, as the cantilever loads can be huge, plus the weight shift being so far aft, you might need buoyancy pods. Considering you've really screwed with the balance of this boat, with the conversion to CC, careful weight estimates should be conducted, to get the CG where you want it. Obviously the tank(s) will need to live where the old engine bay was, possibly farther forward to offset the bracketed outboard. It may be likely that removing the flybridge and other structure will save enough weight to permit the boat to balance out with some minor adjustments.

The LWL clearly shows she was a pretty heavy beast. You've cut away a lot of materiel, so getting the CG close to where it was will be interesting to say the least.

 

For the transom I will be using three 1/2" pieces of composite board, with a layer of 1708 between them. I will probably do 3 layers of 1708 on the final inner skin - all tabbed properly to the hull, in multiple layers. It should be bullet proof, as the transom is very strong as is now. I will be adding addition stringers that tie into the new transom, as well as knees to the existing stringers.

I am not the first one to do this with this exact hull - I've talked to a few other people who have converted these flybridge hulls to open CCs with great success - they kept the boat as an inboard.

For the bracket I was planning on a flotation bracket, that should help with the weight at the stern.

 

The most critical thing you have done is completely redistributed the balance of he boat. Much weight of the cabin and motor have been removed from forward and you are adding the weight of a big outboard well aft of the original stern. This is bound to have an effect on both running and static trim. She is going to sit bow high and run the same way unless some other mods are made.

To start, I would do a weight balance study of the original as well as you can do it. Then do the same calculation of the new boat as you have laid it out. Depending on how the boat ran before , you can make an estimate of the new trim. I suspect it will turn out to be a pretty high trim angle which will not be too good and delay reaching plane, perhaps in spite of the lighter weight of the new boat.

Just saw that you are looking at a flotation bracket which is good.

One thing to look at is that the outboard bracket can be made as a hull extension where the considerable volume of the bracket can add positive buoyancy to the stern. The waterline may not be lowered so much with the new, lighter weight and the chines may still be well immersed which is important for stability.

There may still be a requirement for trim tabs and you can also build in a hook on the bracket bottom for running trim.

Of course, this is all conjecture based on photos and a 60 second analysis by an amateur powerboat designer.

 

Well my plan is to float it after the stringers are all in, and a bracket is mounted. I can simulate weight with drums full of water - this well give me a rough idea of how she will sit. My biggest concern was the keel effecting performance of the hull with an outboard. It seems it will not be a game changer after speaking to some people and suggestions above.

 

Yes, I figured trim tabs is a must ...

 

With a full width flotation bracket, reinforcement of the transom may not be what is needed. Better to extend full height stringers from the current engine beds to carry the load as well as attaching bracket to the hull sides. Trailering loading is probably a bigger issue than actually running on the water. Doing the reinforcement this way allows an opening door in the transom for using the bracket as a swim and loading platform.

 

What type of composite board are you talking about?

 

Penske