What determines a boats max HP rating?

[eavega]

Hello all...

So, this is a follow-up to a question I posted on the Outboard Motors thread. For a quick synopsis of that: I have a Valco 14' aluminum v-hull boat, and given the age of the hull (~50 yrs) and my budget, I tried to power it with an old 10 HP seahorse. I found this setup woefully underpowered and wondered if there was something wrong with the motor. The responses I got on that thread basically boiled down to 1. get the weight distributed correctly on the boat so it was not stern-heavy, and 2. I was right on the cusp of what it would take to get that boat to plane. The boat is rated for 25 HP max, and someone there told me the rule of thumb was 50% of the max rating is needed to plane the boat.

I have since looked at other aluminum v-hull fishing boats, and found that although the construction looks almost identical to my boat, A lot of them are rated for much smaller motors and weight (usually in the 10-15 HP range, with max weight of 500-600 Lbs). This is probably why I was told by everyone that a 10 HP motor should be fine on my boat.

So, my question is; what actually determines the max HP (and weight) of a boat? Is it materials, construction, design, etc? About the only real difference with my boat and other similar boats is that it is a bit wider across the transom than some others.

I was just curious, because the cost of another boat of this size, and the cost of a bigger outboard motor is roughly equivalent, and I was trying to decide which route to take.

Rgds

Eric

 

[JB]

Re: What determines a boats max HP rating?

First are the boats dimensions. . .length, beam, depth (including transom height).

Then there is the boats weight, usually dictated by the design and construction. Strong, rigid boats are rated higher than lighter weight construction.

I don't know where you got the "50% of rated HP" statement. We here usually say not less than 70% of rated HP. That would call for an 18-20HP engine. . . a whole lot stronger than any 10HP, which is only 40% or the rated 25.

I think a mature JohnnyRude 18 or 20 ( I am thinking 1970s) would make your Valco perform very well.

 

[eavega]

Re: What determines a boats max HP rating?

I don't know where you got the "50% of rated HP" statement. We here usually say not less than 70% of rated HP. That would call for an 18-20HP engine. . . a whole lot stronger than any 10HP, which is only 40% or the rated 25.

Again, I was clueless about this, and that was given to me as a response on a previous thread. I didn't take it as fact, but it at least made me aware that there is a relationship between the max HP rating for a boat, and what it would take to make it plane.

I think a mature JohnnyRude 18 or 20 ( I am thinking 1970s) would make your Valco perform very well.

Funny you shold mention that, I found a guy who was willing to trade my running 1960 10 HP Seahorse for a "mostly running" 1970 20 HP seahorse I'll be picking it up on Saturday, then its back to the Johnson/Evenrude outboard boards as I get that one running correctly!

 

[Silvertip]

Re: What determines a boats max HP rating?

Years ago I owned two 14 foot open fishing boats (row boats is a more common term). One was very light weight so my wife and I could easily throw it on top of the car. The other was hardly a lightweight and was towed on a trailer. So even though the boats appear nearly identical, thickness of material, amount of bracing, amount of wood, and number of fasteners (rivets) can make a tremendous difference in weight. Think about the transom alone. Aluminum boats typically have a full sheet of aluminum across the back and marine plywood may be sandwiched between another sheet of aluminum on the inside of the boat. Another boat may have the outer skin but just a narrow piece of plywood across the back and no inner skin. That alone can account for approximately 15 - 20#. Wood seat tops vs bare aluminum also adds weight. To compensate for weight, one needs more power. Obviously that is proven by running with one person in the boat and then running with two or three. The boat will simply not be as fast.

 

[Willyclay]

Re: What determines a boats max HP rating?

When JB speaks, I listen. He is an expert, I am not. However, when I needed to determine the horspower capacity of my old boat, I was led to the USCG regs for boat builders since that is the law. If you put "33CFR183.53" into your favorite search engine, you should find some interesting reading about how the different aspects in boat hull design affect horsepower capacity. After identifying the factors for your hull, you can then do the numbers. This will give you an upper limit for making decisions about how big a motor to use. Then, you will need to consider "old" versus "new" horsepower. The current industry standards, which have been in effect since 1985, measures horsepower ratings at the prop-shaft of outboard motors. An outboard motor manufactured prior to that year may have had it's horsepower rating established at the powerhead without friction loses for the drivetrain OR simply by the marketing department of some manufacturers. Good luck and keep us posted on what you do!

 

[eavega]

Re: What determines a boats max HP rating?

Years ago I owned two 14 foot open fishing boats (row boats is a more common term).

Come on! Don't ruin the illusion that I actually have a fishing boat by calling it a rowboat!

 

[steelespike]

Re: What determines a boats max HP rating?

You'll surely do well with a 20 but my experience with the 10 hp J/E is it will plane pretty easily with the tank forward and a 200 pounder in the stern seat.
We used to have 14ft Crestliner rated for 35 hp w a 10 Johnson with 2 of us kids on the rear seat (about180 lbs) one steering it would plane even with full trim and then we would see how much the bow would bounce.Properly trimmed we did easily 19 or 20 mph.