Powering Down to 20hp

Liopleurodon · Jun 20, 2011

I have another thread started on general noob questions, but there's one area I'm looking to focus on in particular. That would be installing a 20hp motor on a boat that's intended for something much bigger. Specifically a fiberglass V or tri-hull with 6-8 person capacity.

The main reason would be to comply with horsepower limitations on certain local lakes. I am curious as to whether there would be any notable efficiency improvements due to the reduced weight, or losses due to the engine pushing something much larger than it was intended for. Particularly, would it stand a chance at all of planing? Or from another angle, what's the biggest boat/cargo that a 20hp motor could plane?

I'm also wondering if this would have a large effect on prop selection. I would think the pitch should be different depending on the expected speed of the boat relative to the prop RPM.

I'm not concerned much with speed, but am concerned with economy and range. Thanks all for the expert input and patience.

H20Rat · Jun 20, 2011

Re: Powering Down to 20hp

Liopleurodon said:
I have another thread started on general noob questions, but there's one area I'm looking to focus on in particular. That would be installing a 20hp motor on a boat that's intended for something much bigger. Specifically a fiberglass V or tri-hull with 6-8 person capacity.
...
Particularly, would it stand a chance at all of planing? Or from another angle, what's the biggest boat/cargo that a 20hp motor could plane?

not a chance, EVER, of getting a boat rated at 8 people to plane with 20hp hanging on the back... Depends on hull, but you would be talking under 10 mph or so. In you other thread, 'kicker' had came up. That is just an secondary outboard mounted on a side bracket, for the exact purpose you talked about. (engine hp restricted lakes)

5150abf · Jun 21, 2011

Re: Powering Down to 20hp

Ya a 20 will move the boat but not much more, you don't say what it is rated for but the general rule is no less than 70% of rated HP.

It would probably get pretty bad mileage, yes it is a small engine but it is pushing a large load so it will be working harder and burning more gas.

I would say the biggest boat a 20 could plane would be a 14' jon and you would need to run a pretty low pitch prop because of the load.

Home Cookin' · Jun 21, 2011

Re: Powering Down to 20hp

if you don't have the boat already, your best option is a perfectly flat bottom like a carolina skiff J series; they also have very large passenger capacity for their length (per the plate). They are also extremely light. A tri-hull would be a pig. The boat will plane fine with 1-2 people in it; do you typically carry a crowd of 6-8?
You may be better off with 2 small boats properly rigged. It's our solution at our hunt/fish/beach camp where we have large crowds and skinny water.

Liopleurodon · Jun 21, 2011

Re: Powering Down to 20hp

Very helpful info, thanks! Settles the question of whether a 20 could possibly be a primary engine even on the small end of the boats I'm considering, and the kind of speed a kicker will give on the smaller lakes.

ezmobee · Jun 21, 2011

Re: Powering Down to 20hp

Lake Marburg perhaps?

A 20 isn't going to use much gas in the grand scheme of things even if it's pushing a big load so I wouldn't worry much about that.

canuckjgc · Jun 21, 2011

Re: Powering Down to 20hp

Why not get a 10hp kicker? Use it to comply with low hp lakes, and the main engine to use the boat elsewhere.

Liopleurodon · Jun 22, 2011

Re: Powering Down to 20hp

I have a feeling that if this were possible it would have been done already, but I'll ask just in case...

Is there any way to reduce the power capacity of a motor to legally become a 20hp or lower motor, while still being the same engine? I'm thinking some throttle limiter that has a lockout for which you leave the key at shore to prove it can't be switched off. Or maybe a carbeurator? Might need to fly a flag reading "20hp limited" to avoid being stopped all the time. Somehow I'm thinking the larger prop would be better to push the larger boat even at lower speeds, but I don't really know.

smokeonthewater · Jun 22, 2011

Re: Powering Down to 20hp

nope.....

H20Rat · Jun 23, 2011

Re: Powering Down to 20hp

Liopleurodon said:
Is there any way to reduce the power capacity of a motor to legally become a 20hp or lower motor, while still being the same engine? I'm thinking some throttle limiter that has a lockout for which you leave the key at shore to prove it can't be switched off. .

Well, kindof... Its fairly common for the hoods used on outboards to be the common size, so you can swap fairly easily. Or worst case, just clean off your outboard hood and buy an OEM sticker kit for the smaller engine. This works well if you have a 15 hp stuffed into a 9.9hp hood, as lots of lakes have an under 10hp restriction. (hence lots of 9.9 engines)

I have a feeling someone might notice if you rebadge a 125hp outboard as a 20 hp!

As far as limiting the throttle, you would have to convince whatever game warden that stops you about that, which I doubt you would have much of a chance doing. HP limits aren't just about speed, they also limit noise and pollution. Using a 125hp 2 stroke to troll at idle speed puts out A LOT more pollution than a little 10 hp running at 1500-2000 rpm or so for trolling speed.

Liopleurodon · Jun 26, 2011

Re: Powering Down to 20hp

Another scenario to try:
Is there any boat out there that can do 20mph with 20hp and 4-5 people (say 750lb of passengers), including a steering wheel, bow seating, and comfortable rear seating (ie not just the flat seats of a typical jon boat)?

smokeonthewater · Jun 26, 2011

Re: Powering Down to 20hp

my gut says 10mph with that much of a load.... for that many people and lounge seating, just get a toon and forget speed

H20Rat · Jun 26, 2011

Re: Powering Down to 20hp

Liopleurodon said:
Another scenario to try:
Is there any boat out there that can do 20mph with 20hp and 4-5 people (say 750lb of passengers), including a steering wheel, bow seating, and comfortable rear seating (ie not just the flat seats of a typical jon boat)?

Outside of some exotic race hull, there is no typical boat on the market that can possibly get 20 mph with 5 average size people on board. I agree with the previous post, 10 mph with 5 people is more reasonable, and even that might be if you go before everyone has lunch.

With that said, I do know of 1 boat that could do 20 mph with a 20 hp hanging on the back... Creature comforts and sea handling, non-existent...

Liopleurodon · Jun 29, 2011

Re: Powering Down to 20hp

Ok, last whack at the horse before conceding defeat...
Given:
700lb of people - 2x 200lb and 2x 150lb.
4 stroke 20hp motor with steering up front
A wide range of hull shapes and materials - not a crew boat but maybe an aluminum tunnel or catamaran

Is it remotely feasible to travel modest distances (say 5-10mi) upstream in strong current? Maybe not 20mph against 10mph current, but 15 against 7?

smokeonthewater · Jun 29, 2011

Re: Powering Down to 20hp

In the type of boat you asked about before, I don't think so.... 8-10 against 7

keep in mind that whatever weight you put into the boat displaces water.... 700 lbs of people 500 lbs of boat and 100 lbs of engine displaces 1,300 lbs or about 165 gallons of water.... now since you won't get on plane you must drag that mass through the water and that's a lot of drag

Your BEST chance would be in a lightweight flat bottom john boat, no wood floors, no heavy coolers, and minimal gear. .... But if you want room and comfort then you are likely sol...

southkogs · Jun 29, 2011

Re: Powering Down to 20hp

My family used to own a pontoon when I was a kid. It was an early 70's Crest with an Evinrude 15hp on it. The reason I mention the 70's Crest was because it was one of the early types of pontoons - two cans, a deck, rails and a steering wheel. We could pile a bunch of people on lawn chairs on that thing and put around for a couple of good hours. The 15 would have to work hard if we had it loaded down (and burned more gas than normal), but it could run in a foot or two of water and you didn't really feel cramped. It actually sounds kind of like what you're looking for - if you can get past having to trailer it.

The newer pontoons have much more deck gear installed on them. They're certainly nice and comfy, but one of the old plain Jane deals may fit the bill.

Also, I would think that prop size and design on a 20hp is significantly different than that of a 70hp or 100hp. Meaning, not only do you lose the shear "beef" of the motor but also the effect of a propeller sized and pitch more suited to the boat.

Hope you find what works for you.

mark mcj · Jun 29, 2011

Re: Powering Down to 20hp

Maybe scouting around the local launches, mooring or marinas might give you idea of what you could use.

Liopleurodon · Jun 30, 2011

Re: Powering Down to 20hp

smokeonthewater said:
In the type of boat you asked about before, I don't think so.... 8-10 against 7

keep in mind that whatever weight you put into the boat displaces water.... 700 lbs of people 500 lbs of boat and 100 lbs of engine displaces 1,300 lbs or about 165 gallons of water.... now since you won't get on plane you must drag that mass through the water and that's a lot of drag

Your BEST chance would be in a lightweight flat bottom john boat, no wood floors, no heavy coolers, and minimal gear. .... But if you want room and comfort then you are likely sol...

Thinking about the Jon boat scenario - is there a weight threshold that I would need to figure out how to stay under? Don't need much room as long as everyone's friends, and could probably figure out lightweight seating.

A small pontoon doesn't sound like a bad idea either.

At this point I'm definitely talking about a different boat than in my other thread.

lncoop · Jun 30, 2011

Re: Powering Down to 20hp

John boats are awesome, but that's not what you want. In order to get the capacity you need you'd have to have an eighteen footer minimum, and in order to have room for all the bottoms it would have to be wide open with tiller steer, which is a rare setup and would likely have to be ordered, which means brand new pricing from a dealer, which means OUCH!

You need an 18'-20' pontoon boat with forty horseys and PT&T. Easy to drive, easy to tow, easy to launch and load, easy to pay for. Take $1000 of the money you save and buy a 14' john boat with a 9.9 and go forth and be happy.

That is all.

smokeonthewater · Jun 30, 2011

Re: Powering Down to 20hp

To give you an idea my dad has an 18' alum bass tracker (John boat) with a 65 and I'd tops out at 38 w two people in it..... It does have floors and such so it weighs more than an open boat but it requires about half throttle to get on plane..... I don't know if 20 hp would plane it or not tho.

I think Incoop REALLY has it... You need two boats.

Powering Down to 20hp

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