Transom length - Max horse Power

[Arkman]

I'm wondering how the transom length affects maximum horse power. If I have a short shaft transom rated for 40 hp. Can you safely install a larger motor if I converted the BOAT to a long shaft transom. I understand you have to you the proper size motor for the boat, short or long shaft. I'm not talking about going for 40hp to 120hp or anything. Just like 40hp to 50hp or something like that.

I'm not planning on doing it, just wondering if there is any change. Now I realize the max hp on the id plate would still read 40 hp so I suppose technically you might be able to get in trouble or something.

 

[tallcanadian]

Re: Transom length - Max horse Power

You have to measure your tansom length. If you have a 15" transom then you should have a short shaft motor. A long shaft motor on this transom will be too long, 20" and will produce a lot of drag. Probably you will get water in your splashwell. You would have to add approx 5 inches to your transom length. Your cavitation plate on your outboard should be even or a little higher than the bottom of our transom. I hope this helps.

 

[ajgraz]

Re: Transom length - Max horse Power

I think the OP is asking, if he makes the transom taller, will it be able to take the added torque from the longer lever arm of a long-shaft motor of similar or slightly higher hp?

Technically, I'd say yes, as long as you made a good, solid, well-supported transom. Heck, anyone replacing or repairing a transom--extended or not--is gonna beef it up to better than original anyway, right?

Legally, as long as the hp rating on the plate is not exceeded, go for it.

 

[tallcanadian]

Re: Transom length - Max horse Power

I think the OP is asking, if he makes the transom taller, will it be able to take the added torque from the longer lever arm of a long-shaft motor of similar hp?

Technically, I'd say yes, as long as you made a good, solid, well-supported transom. Heck, anyone replacing or repairing a transom--extended or not--is gonna beef it up to better than original anyway, right?

Legally, as long as the hp rating on the plate is not exceeded, go for it.

Your right, AJ, I've done this and had great results. Not sure what's out there for transoms....more short shafts compared to long...not sure on that one. It seems more time than not that the transom has to be raised.

 

[Bondo]

Re: Transom length - Max horse Power

I'm wondering how the transom length affects maximum horse power.

Ayuh,... What is the highest horepower available with a 15" case,..??

 

[Bob_VT]

Re: Transom length - Max horse Power

Transom height IS NOT in the formula for max HP rating.

Short answer.... NO a deeper transom will not increase the HP capacity

 

[Arkman]

Re: Transom length - Max horse Power

Transom height IS NOT in the formula for max HP rating.

Short answer.... NO a deeper transom will not increase the HP capacity

Fair enough. What is in the formula? Just out of curiosity.

 

[Bob_VT]

Re: Transom length - Max horse Power

http://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf

 

[Arkman]

Re: Transom length - Max horse Power

Thanks bob. According tho that pdf if my transom height is 15" then my max hp would be 35 (unless I did something wrong, its math so....probably). According to my tag mine is 40. Could be I just rounded in the wrong spot or something. Now If I raise my transom to a 20" transom then I would be at a maximum of 60hp.

Certainly not something I'd put on MY boat. Thats just an accident waiting to happen!