Extending the transom for max HP

[oneoldude]

On my project boat the transom is 19" deep rather than the standard 20" deep. The cavitation plate of the motor is therefore 1" below the bottom of the boat. How does that affect performance?<br /><br />If I do the USCG calculation for max HP for the hull, that 1" reduces the USCG reccomendation by almost 50% vs the reccomendation for 20" transoms! That 1" difference relegates the boat to motors that are appropriate for 15" transoms! It seems the USCG is a bit draconian on this issue. Thoughts?<br /><br />The transom (where the motor mounts) is 5" lower than the rest of the stern so there is a lot of room to add another inch to the transom and therefore almost double the USCG max HP rating for the boat. Is it a good idea to extend the transom up by that 1"?<br /><br />If it is a good idea to make the transom 20" deep, what is the proper way to do it without removing the existing transom that is perfectly sound and strong? I know a jack plate can raise the motor but the transom will still be 19".<br /><br />The stern of the project can be seen at the URL below.<br /><br />Thanks.

 

[ED21]

Re: Extending the transom for max HP

What was the original mfg. max hp for the boat?<br />What does it calculate to be?<br />What would you like to put on it?<br />No suggestions, just curious.

 

[oneoldude]

Re: Extending the transom for max HP

FWIW<br /><br />Manufacturer Rating = 50 HP<br />USCG Calc. 19" Trans = 45 HP<br />USCG Calc. 20" Trans = 85 HP<br />My motor 1980 Rude 70 = Approx. 60-65 HP at prop

 

[ED21]

Re: Extending the transom for max HP

I don't see why raising the transom 1" would be difficult. It could be as simple as adding a p.t. 2x2. <br />Most of the time the motor bracket doesn't rest on the transom. The bolts through the transom hold it on. You would have do drill at least 2 new holes. They may not even be in the added piece.<br />I'm sure you will think of a nicer & better looking way than a 2x2 though.<br />I would also add, for 1" who cares.<br />It doesn't sound like a 70 will be over powered.

 

[thehermit]

Re: Extending the transom for max HP

Can you bump up the motor one set of holes and be done with it?

 

[JasonJ]

Re: Extending the transom for max HP

If you have to replace the transom during your rebuild, you can raise it any dimension you want. In fact, even if your transom is in decent shape, I recommend replacing it with the desired dimension. You will have maximum strength that way. An alternative is to add wood to the top to create the desired dimension, then use fairly thick aluminum plate between outboard bracket and transom to distribute force better. Otherwise that added wood could be a weak link in the equation. Good luck...

 

[JasonJ]

Re: Extending the transom for max HP

One thing I would also like to add though. Your deadrise is minimal. As a result, if you raise the motor too much, you will ventilate in tight turns because the boat will want to lean and the prop will lose bite easier due to the height. This is something to consider, because if you build in too much height, and have to adjust the motor back down, you could end up cutting the transom back down. Like TheHermit said, if you can raise the motor with existing holes, try that first, you may find that it is already at the optimal height for your hull design. Good luck...