What would be a good Minimum HP rule-of-thumb?

Hi guys

I have a good-running 50 HP evinrude that I took off of a small fiberglass tri-hull that was going to require a full deck/transom/stringer rebuild. I was intending to do the rebuild work, but before I lay out money for a rebuild, I was going to explore the possibility of just buying another hull. I've seen a few, but what I keep finding is that they are bigger than the boat I took this 50 HP off of, and all are rated maximum HP higher than 50 (I don't know what the original hull was rated for, since someone removed the capacity plate). My question: Is there a reliable rule-of-thumb for the smallest motor you can put on a boat based on its Max HP capacity? I've heard from different people you need bare minimum 50% of max HP to plane a properly trimmed boat, I've heard 70% will move you but won't break speed records. I won't needing the boat to have a lot of speed to it, since it will mostly be used for fishing or just slow cruising, but I also don't want to be underpowered to the point of just plowing through the water at less than 10 MPH. Any advice would be appreciated.

Rgds

-Eric

Re: What would be a good Minimum HP rule-of-thumb?

70% of max hp is a good rule of thumb to at least be able to plane and cruise while using less fuel. If the rig is going to be heavily laden I'd go bigger.

Re: What would be a good Minimum HP rule-of-thumb?

Tough one, because different hulls plane differently.

On a V, you need 70% give or take a few % points.

Now, contrast that with a flat-bottom boat...

On a flat bottom, like a skiff... geeze, they plane with a light breeze! So, it'd be something less than 70%. Heck, 50% or less might work.

There are guidelines and guesstimates, but you have to consider the boat's layout (how stern heavy is it?), overall weight, length, beam, and the hull design, and whether power trim will be set up.

Re: What would be a good Minimum HP rule-of-thumb?

What kind of boat are you looking for? There are plenty of boats that perform well with a 50 hp, but they may not be what you're looking for. You can do especially well with some of the aluminum models or classic designs, if you can find one in great condition.

If you can identify the type of boat you want, then you can get closer to finding a good match with your 50.

Re: What would be a good Minimum HP rule-of-thumb?

CATransplant said:

What kind of boat are you looking for?

Click to expand...

I'm looking for something that floats and doesn't need a rebuild!

Seriously, the most versatile thing I have seen for my needs is some kind of small bowrider. The 15' tri-hull the motor came off of is a good starting point; it was small, walk-through windshield, bowrider circa 1970s. The main use of the boat will be for fishing on big lakes, but I want something that has enough deck space to walk around on, enough room for two adults and maybe a kid to fish from or just cruise around, and enough freeboard so I am not getting soaked on a windy day.

Here is a picture of the original boat

Re: What would be a good Minimum HP rule-of-thumb?

I have a very similar boat to the one you took your motor from. It is rated or 75hp but the lake i fish on is quite small and i usually only fish with me or me and a buddy. I have only a 25hp motor on it right now. It will plane just fine if there are only two people in it, but it will plow if i start adding more people, which is fine with me because if my family comes out with me we would only be cruising anyway. so 33% of the capacity plate works for my hull and my desires.

Re: What would be a good Minimum HP rule-of-thumb?

Huron Angler said:

70% of max hp is a good rule of thumb to at least be able to plane and cruise while using less fuel. If the rig is going to be heavily laden I'd go bigger.

Click to expand...

I agree.... and the key i think is what is bolded above.

You most likely can plane/cruise with less HP, but the closer you are to WOT, the more fuel you'll consume. I have a boat (deep V) that is at the 85% and it nicely planes/cruises around 2800-3000RPM, a fair bit short of the max around 5600RPM.

And, as Philster mentioned, the hull design has a big impact on that as well.

Re: What would be a good Minimum HP rule-of-thumb?

Some very light boats can get by with 50% of rated power, but I agree with the rule of thumb quoted above: You need 70% of rated HP for satisfactory all-round performance.

Re: What would be a good Minimum HP rule-of-thumb?

Aluminum hulls can do more with less HP. Just something to think about.

Re: What would be a good Minimum HP rule-of-thumb?

Calculation for HP requirement: wgt of boat in lbs divided by 1 equals required horsepower. Just kidding! Actually I had a 50 hp johnson on the back of my
73 fiberglass starcraft tri-hull and when propped right at 5500 rpm it ran ok.
I now have it on the back of at 17ft aluminum seanymph open bow, walk thru console and it flies. I dont have a speedo or gps to check speed but it made
one heck of a difference. The SeaNymph is rated at 100 hp max but thats way more than this boat needs.

Re: What would be a good Minimum HP rule-of-thumb?

The best rule of thumb is this.

No matter what you buy, you will want more.

People have boats with moters that are at max HP ratings, and want more after they get it.
Eventually, No one has ever had a boat with enough power

Re: What would be a good Minimum HP rule-of-thumb?

Don S said:

The best rule of thumb is this.

No matter what you buy, you will want more.

People have boats with moters that are at max HP ratings, and want more after they get it.
Eventually, No one has ever had a boat with enough power

Click to expand...

Well, not exactly no one. I don't wish for more power. The little 6hp on my 12' aluminum boat is completely adequate for how I use that boat. In fact, I downsized to that boat intentionally, and I'm catching more fish and fishing more often due to its ease of use and suitability to the task.

Re: What would be a good Minimum HP rule-of-thumb?

Folks

Thank you so much for all the replies and advice. I went to a local salvage yard and hooked up with a 15' bowrider hull that is, as far as I can tell, identical to the 15' Chapparal I was going to restore, with the exception that the deck has no soft spots, and the transom was rock solid ( no flex despite my determined efforts, and I didn't hear and "dead" spots tapping with a hammer). Bonus is that I got the hull for less than half of the cost of materials to do the deck and transom restore, and it includes the steering system, which was questionable at best on my original hull. Its a 1976 Yarcraft Yankee. Its rated for a 70 HP motor, so it should had satisfactory performance with my 50 HP.

I'll pick up the hull on Thursday, and will post pictures then

Again, thanks for the information and opinions.

Rgds