Max Horsepower Rating Interpretation

[FishboatWIMI]

Hi All - I've read a lot of threads on here in considering taking on a project boat. Found a lot of helpful info but there is one aspect I have not seen covered, hence this post.

That topic is horsepower ratings on older (1970’s) boats translated to today. I came across a formula that the coast guard uses to set the max hp rating based on length and beam at least for boats under 20’. I’m wondering whether that limit relates more to the weight of the outboard or the power produced.... Here are a few things that led me to this question:

I’m considering restoring a boat from the late 1970’s, which is max rated for 175 hp. A repower is something I anticipate wanting to do as part of this process.

When I look up the engine weight of a 1978 Evinrude 175 hp it is ~385 lbs. depending on exact model. However if I look up the weight of a brand new 4 stroke 175 outboard (I checked Yamaha just as a brand I might choose) and the 2020 Yammy is ~485 lbs, again depending on exact model.

Sticking with Yamaha brand just for my example, I’d have to go down to a new 115 hp to be in the weight ballpark of the 1970’s 175 hp Evinrude.

I have no interest in wanting to overpower the boat and I want it to be safe. However I would like to maximize its speed performance within those bounds. So what does everyone think - if I do repower with a new outboard do I go with the 115, the 175 or do I compromise somewhere in between?

Thanks for listening to my rambling and I appreciate any insight.

 

[chevymaher]

Go with the power you want. Inboards I/O don't have this restriction and their motors are much heavier. Go with max power allowed. The weight difference is nothing.

 

[dingbat]

The max HP calculation deals with hull displacement. It does not take weight into account.

Depending on the boat, an additional #100 in the back of the boat may or may not be of concern at all. Keep in mind your typical battery weighs #40 each....

Worst comes to worst you may need to move some weight forward to compromise for the additional weight

 

[Scott Danforth]

Go with the power you want. Inboards I/O don't have this restriction and their motors are much heavier. Go with max power allowed. The weight difference is nothing.

because the load path is different on an I/O vs an outboard

an outboard is clamped to the top of the transom, the force of the prop is trying to pry the transom off the back of the boat with a long lever arm

an I/O is mounted low to the transom and stringers, much less force on the transom, even with double the HP

 

[FishboatWIMI]

Hi All - I've read a lot of threads on here in considering taking on a project boat. Found a lot of helpful info but there is one aspect I have not seen covered, hence this post.

That topic is horsepower ratings on older (1970’s) boats translated to today. I came across a formula that the coast guard uses to set the max hp rating based on length and beam at least for boats under 20’. I’m wondering whether that limit relates more to the weight of the outboard or the power produced.... Here are a few things that led me to this question:

I’m considering restoring a boat from the late 1970’s, which is max rated for 175 hp. A repower is something I anticipate wanting to do as part of this process.

When I look up the engine weight of a 1978 Evinrude 175 hp it is ~385 lbs. depending on exact model. However if I look up the weight of a brand new 4 stroke 175 outboard (I checked Yamaha just as a brand I might choose) and the 2020 Yammy is ~485 lbs, again depending on exact model.

Sticking with Yamaha brand just for my example, I’d have to go down to a new 115 hp to be in the weight ballpark of the 1970’s 175 hp Evinrude.

I have no interest in wanting to overpower the boat and I want it to be safe. However I would like to maximize its speed performance within those bounds. So what does everyone think - if I do repower with a new outboard do I go with the 115, the 175 or do I compromise somewhere in between?

Thanks for listening to my rambling and I appreciate any insigh

because the load path is different on an I/O vs an outboard

an outboard is clamped to the top of the transom, the force of the prop is trying to pry the transom off the back of the boat with a long lever arm

an I/O is mounted low to the transom and stringers, much less force on the transom, even with double the HP

Thanks for the responses all - this comment by Scott goes along the lines of the concerns I had.

Just to give a few details the boat is ~19.5 feet long with a ~7.5 foot beam, fiberglass hull and closed bow.

Fortunately, I have plenty of time before I have to decide since I’m just starting to plan and budget for this project.

Had another thought - since I plan to replace the transom anyway should I take the opportunity to do something other than the standard marine plywood/fiberglass transom? Maybe add a layer of metal or go with something composite?

 

[racerone]

If you plan on keeping the boat then go with composite materials.-----If you go boating on a wee pond ( warm water ) with hundreds of helpers around then you can cut corners.-------If you go boating in the middle of nowhere use the best materials !-----Saw a boat sink in Lake Huron.-----Folks were lucky they were in the middle of sailboat race course.-----Eight sailboats with experienced folks on board rescued them in minutes !

 

[Sea Rider]

What's the Max passengers allowed with Max HP the boat is rated for ? With how many souls on board do you plan going boating with ?

If you'll be boating with less than the Max passengers allowed, don't need to go for the full HP rated for. It's a matter to prop right the motor to run at least middle to max wot rpm factory stated. A prop maximization will do the trick....

Ca go for a Yamie 150 which cost less than a 200, but both weights 226 Kg each, so will need to move weight forward to compensate the extra aft weight of 4 stroke motors.

Happy Boating​