Wooden Boat transom rating

evinrude20

Seaman
Joined
Jun 5, 2008
Messages
56
Hi all, I have a question regarding the transom of my wooden boat. Its a custom craft shelter top cruiser, 19' with a roof, closed bow. When it was built (57') it had an Evinrude 50, big twin I think? first on the lake that size. Back in the 90s, it got upgraded to a 84' Suzuki dt115, which moved the boat along nice. There is room for about 8 adults comfortably. I have to redo the transom, which i believe is just a couple sheets of 3/4" plywood attached the the frame,(fibreglass outside) the 115 I have is going by the wayside, and I have a Blackmax 135 (88') I can get for free from a relative. I looked and it seems to be about 30lbs heavier and of course the added 20hp. Is this going to be an issue for the boat? Im not an engineer, just want to make sure its not over doing it. Thanks!!
 

zool

Captain
Joined
Aug 19, 2012
Messages
3,432
Re: Wooden Boat transom rating

depending on the width of your beam, you may be ok

19 (length) x 6 (beam) = 114 x 2 = 228 - 90 = 138 hp for non tiller

thats just a simple rough estimate, not sure if it proves legality...
 

evinrude20

Seaman
Joined
Jun 5, 2008
Messages
56
Re: Wooden Boat transom rating

Thanks for the info, I will check the measurements! Is 90 a constant?
 

jbcurt00

Moderator
Staff member
Joined
Oct 25, 2011
Messages
24,829
Re: Wooden Boat transom rating

Thanks for the info, I will check the measurements! Is 90 a constant?
Yes, 90 is a constant.

Hopefully this guide will help you calculate the maximum amount of H.P. your boat can be insured for.
- This is to only be used as a guide.
- You can round your final HP number to the closest multiple of 5. (31 HP would round to 30 HP. 33 HP would round to 35 HP)


Step One - Compute your Factor

A. Length of Boat x Transom Width (measure in feet: 14, 15.5,16, etc).



Step Two - Compare your factor to H.P. rating

A. 0-35 = 3 hp
B. 36-39 = 5 hp
C. 40-42 = 7.5 hp
D. 43-45 = 10 hp
E. 46-52 = 15 hp



Step Three - If your factor is over 52 (Choose only A or B)

A. With Remote Steering (steering wheel) and at least 20" transom height

Maximum HP = (Your factor x 2) - 90

Example: 19' 6" long , 7 feet wide = (19.5 x 7 x 2) - 90 = 183 (Round up in multiples of 5. This boat's max is 185 hp).


B. With Tiller Steering (hand or extension) or less than 20" transom height (Choose only 1 or 2)

1. Flat bottom, or hard chined boats (hull has hard angles)

Max HP = (Your factor x 0.5) - 15

2. Soft chine, rounded hulls

Max HP = (Your factor x 0.8) - 25
 
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