Transom Ratings?

[searaynoob]

I don't understand horsepower ratings for boats...you see all these tiny little bass boat transoms with like 225 horse on it, and then like my crestliner I have a huge transom with a rating of 115...I don't get it how a transom or boat is rated for horse power. Anyone have insight on this???????????????

 

[Georgesalmon]

Re: Transom Ratings?

There is a formula used on outboards. The width of the transom has a lot to do with hp rating. Bass boats typically have really wide transoms thus the high HP. Just because the formula says one thing many manufacturers will downgrade the HP rating to what they feel is safe also. There is no formula for pontoons and I/O's, its all up to the manufacturer.

 

[roscoe]

Re: Transom Ratings?

Its not how big the transom is, its how you use it... I mean, its how its made.

A 2x14 plank bolted to the hull is not as strong as 3" of plywood, wrapped and coated in fiberglass, and reinforced by 3 knee braces, and double width stringers.

 

[Rancherlee]

Re: Transom Ratings?

There is a formula used on outboards. The width of the transom has a lot to do with hp rating. Bass boats typically have really wide transoms thus the high HP. Just because the formula says one thing many manufacturers will downgrade the HP rating to what they feel is safe also. There is no formula for pontoons and I/O's, its all up to the manufacturer.

Actually there is a Coast Guard formula for pontoons now, its based on cubic feet of flotation and the length/diameter/number of the pontoons. Crestliner and Lund are known for down rating "cheaper line" boats. My parents 1750 Lund Adventure Fisherman has the same IDENTICAL hull/transom as a Pro V of the same year and they are rated for a 125 and the Pro V a 150

 

[belairbrian]

Re: Transom Ratings?

First they are not transom ratings. They are hull ratings. The formula includes transom width, transom height, steering location, hull design,and boat length.

For example two otherwise equal hulls one with steering console one with tiller, the tiller will have a lower hp rating.

my 2010 tracker actually has two ratings a hp rating and an engine weight rating. They are on two separate plates.

 

[belairbrian]

Re: Transom Ratings?

Actually there is a Coast Guard formula for pontoons now, its based on cubic feet of flotation and the length/diameter/number of the pontoons. Crestliner and Lund are known for down rating "cheaper line" boats. My parents 1750 Lund Adventure Fisherman has the same IDENTICAL hull/transom as a Pro V of the same year and they are rated for a 125 and the Pro V a 150

I wonder if the layout of the deck affects those two boats. Could it be weight distribution based on the seating affected the ratings, like remote and tiller steer do on a regular hull?

 

[Home Cookin']

Re: Transom Ratings?

when you are looking at matching a motor with a transom there are many factors--will the weight of the motor sink it just sitting there? Or break the transom? Is the transom strong enough to take the force against it? Is the transom wide enough to float the motor?

One thing to remember on transom strength is that a boat on a plane puts little resistance against the motor. So that big motor pushing a flat-bottom boat isn't bothering the transom much, but it would on a pig of a boat plowing through waves.

But all that is more a question for design rather than safety ratings.

 

[searaynoob]

Re: Transom Ratings?

Ok I was just wondering because I have a 1977 Apollo 660, It is a 17' boat transom width is 6' and height at the outer most parts is 24" and am planning on keeping the 140 horse motor that is on it there after rebuilding the transom. It is rated for a 115 and the bolts from the motor pulled through the fiberglass so I plan on knee braces a solid white oak transom with new thicker stringers and a solid fiberglassed floor...this will be a tubing and fishing boat which is why I want to keep the 140 horse on there!! Let me know what you guys think.

 

[gbrowne9]

Re: Transom Ratings?

i have a 1973 apollo 660 but in canada they call it a crestliner muskie 17. the boat was rated at 125hp in 1973 and i have had a 1977 150 tower of power on it, 1992 suzuki 150 on it ,a 1989 115 merc on it and a 1986 135 on it . the suzuki was the fastest but broke the original transom,it did over 60 mph. the tower of power did 47, the 115 does 43 and the 135 v6 does 52. the problem is the 135 uses twice the gas of the 115,so the 135 has to go.just my 02 cent .i think the 140 will rock.

 

[roscoe]

Re: Transom Ratings?

2" of plywood are going to be a lot stronger than a chunk of white oak.

 

[searaynoob]

Re: Transom Ratings?

2" of plywood are going to be a lot stronger than a chunk of white oak.

I have heard different on this...everyone says its stronger. But I have a very good friend of mine that has the same boat and has had a solid white oak transom and it has held up very well for 10 years. White Oak is one of the hardest woods there is and minus the expansion that someone will mention, I don't see any reason not to use it.

 

[ondarvr]

Re: Transom Ratings?

Making it strong enough to hold a 140 isn't an issue, plywood and glass is fine, what do you think they use to make transoms for much bigger boats that use twin 225s. Plywood is a better product for an application like this, but if white oak makes you feel better it will work.

Over powering is a different topic.

 

[searaynoob]

Re: Transom Ratings?

Making it strong enough to hold a 140 isn't an issue, plywood and glass is fine, what do you think they use to make transoms for much bigger boats that use twin 225s. Plywood is a better product for an application like this, but if white oak makes you feel better it will work.

Over powering is a different topic.

Previous post on motor size solves that for me. but I think I may use plywood now but how would you add strength to that transom without adding too much extra weight?
knee braces?? Always wondered how they do put twin motors on the back of a boat without tearing off the back of the boat.....

 

[NSBCraig]

Re: Transom Ratings?

Those bass boats are 20' or more, which gets them out of the overpowering regulations.

 

[metriccrescentwrench]

Re: Transom Ratings?

Those bass boats are 20' or more, which gets them out of the overpowering regulations.

Yep, that's a bit of a mystery. A 19 ft outboard has a certain max hp rating. Make the same boat a foot and a half longer and there's no limit. Plus there's no limit on I/O's of any size, just outboards.

It is rated for a 115 and the bolts from the motor pulled through the fiberglass so I plan on knee braces a solid white oak transom with new thicker stringers and a solid fiberglassed floor...this will be a tubing and fishing boat which is why I want to keep the 140 horse on there!! Let me know what you guys think......... White Oak is one of the hardest woods there is .....

I'd rebuild the transom the way it was intended to be built, with marine grade plywood. 'Hardness' and 'strength' are two different properties, just because a material is 'hard' doesn't necessarily mean that it's 'strong'. The reason a good quality plywood is stronger is because it is laminated with the plies oriented in specific directions to give it the desired properties.

And I'd consider going with whatever size engine the boat is rated for, if nothing else to avoid any possible hassle from law enforcement or insurance. A 115 ought to be fine on that boat.