Pairing engine size with boat size

[Thirsty Endgrain]

Hello all, I have a 17 foot Glasspar Seafair sedan with a 90 horse two stroke Johnson on the back. I would really like to upgrade the engine for the best combination of efficiency and power, the boat came with a 75 horse Johnson that ended up being dead, and I was able to find the 90 to put on the back. Right now the boat tops out at around 33 to 35 mph. I don’t know how much the boat weighs full of fuel and gear, I need to look up the dry weight which I remember being somewhere around 1500 or 1700 pounds, the engine is around 300, and the fuel and gear probably add an additional 500 or so pounds. So all said and done it probably weighs somewhere around 2400 pounds? How do I figure out What engine size will produce the best efficiency? Would going back down to a 75 hp tend to increase or decrease efficiency? Would going up to 115 hp tend to increase or decrease fuel efficiency? I'm not as concerned with top speed, anywhere in the high 20s is probably enough for me. I also understand that propeller choice will have something to do with it. I’m in my research phase as I want to upgrade to a four stroke, but I want to make sure I make the right choice. Thanks in advance.

 

[briangcc]

What does your capacity plate state for HP/weight? That's going to be the determining factor. Then I'd rig a 2 stroke at max as I've yet to hear anyone utter the words...."Gee, I wish I bought a smaller engine".

As for the 4 stroke - your boat wasn't designed to carry the extra weight, it will be stern heavy. I'd be very cautious of going that route.

**I loved my Honda 4 stroke so I have nothing against 4 strokes but they are heavier than comparable 2 strokes. The difference between my Force 2 stroke and the Honda 4 stroke was around 180lbs. That alone sunk the boat in the water about 2" give or take. This put my splashwell drains at water level.

 

[Thirsty Endgrain]

Thanks, as near as I can tell from my research this thing was designed for a max 150hp 2 stroke outboard. That's appx. 390 lbs.

My dream engine would be a Suzuki DF90, which weighs in a appx. 340 lbs. So weight wise I think I'm OK sticking with anything in the 90-115 horse arena.

Yeah I likely wouldn't want to downsize the engine, but I'm trying to figure out what's the most fuel efficient option for my combo.

Looked a bit harder and the boat dry weight is appx 850 lbs, I was way off. So total of appx 1400-1500lbs engine and gear.

I know the easiest way towards fuel efficiency is to only have what I need onboard, but since I'm looking for engines I'm trying to figure out how to make that choice...

 

[dingbat]

How do I figure out what engine size will produce the best efficiency? Would going back down to a 75 hp tend to increase or decrease efficiency? Would going up to 115 hp tend to increase or decrease fuel efficiency?

Here is the basic math…your boat takes X HP to move at a given rate of speed. It takes roughly 2,500 Btu to generate 1hp.
This does not change if you go with a 75 or 115 hp motor.

What does change efficiency is the combustion technology. The difference between a carbureted two stoke and an EFI 4 stroke is considerable.

Some will people argue that WOT fuel usage is compatible, but who runs WOT all the time? Most put 4 stroke fuel usage improvement in the 20-30% range.

Personally, nothing worst than an under powered boat. Try to stay within 75% max. Hp rating if your check book and transom allow it

 

[Thirsty Endgrain]

That is some great info right there, thank you!

 

[JimS123]

What does efficiency mean? Fast speed? Low fuel usage? Running the engine at optimum "performance"?

To me, it means all of the above. I have 3 outboard powered boats. Top speeds are 26, 35 and 53. The fastest ones have similar mpg, and the slowest (an oldie) sucks gas like crazy.

I run performance curves and determine the best fuel efficiency. That's where I run. The fishing boat runs slower than the family boat, but I don't care. The oldie goes to the classic boat shows each year, and that's about all, so I could care less how much gas I use.

IMHO, I would look at specs and see what the weight of the max 2-stroke would have been, and then buy a new 4-stroke at the same (+-) weight. But then again, if it ran a little slower I wouldn't be concerned.

After running I/Os and 2-strokes for 50+ years, the only boat I will ever own now will be powered by a 4-stroke OB.

 

[Thirsty Endgrain]

What does efficiency mean? Fast speed? Low fuel usage? Running the engine at optimum "performance"?

IMHO, I would look at specs and see what the weight of the max 2-stroke would have been, and then buy a new 4-stroke at the same (+-) weight. But then again, if it ran a little slower I wouldn't be concerned.

After running I/Os and 2-strokes for 50+ years, the only boat I will ever own now will be powered by a 4-stroke OB.

Seen gas prices lately? Wondering about best combo of fuel efficiency to motor weight that my boat can handle. To your second sentence, I did look that up, posted above. And yes, I'm trying to get to that point, to have a good reliable quiet clean efficient 4 stroke outboard.

I read some comparison yesterday (will try to find) that compared the carbureted vs the EFI mercury four strokes. The tests seemed to suggest that the EFI was more fuel efficient when you factored for fuel usage not at the same RPM's, but at the same boat speed of their test boat.

I've seen trash talked about EFI systems on some of the modern offerings, but I still don't know enough yet to have an informed opinion on this matter. Some say EFI systems are more expensive/complicated, though I'm sure some of this is brand/motor specific. I'm trying to get in and out of the weeds here, ideally with a new-ish motor to get me to the fishing grounds...

 

[Scott Danforth]

Fuel is still the cheapest part of owning a boat.

 

[jimmbo]

At minimum, I would put an Engine meeting the boats max rating.

 

[JimS123]

I've seen trash talked about EFI systems on some of the modern offerings, but I still don't know enough yet to have an informed opinion on this matter. Some say EFI systems are more expensive/complicated, though

Remember annual car tune-ups? I.E, plugs, points and condenser. Then came electronic ignition on carbed engines, now EFI.

Efficiency is one thing. But even more important is reliability. Sure the new systems are complicated and require a trained mechanic and a computer. But the fact that they simply run and run and run (at least in my experience) is what makes it all worthwhile.

 

[Scott Danforth]

Remember annual car tune-ups? I.E, plugs, points and condenser. Then came electronic ignition on carbed engines, now EFI.

Efficiency is one thing. But even more important is reliability. Sure the new systems are complicated and require a trained mechanic and a computer. But the fact that they simply run and run and run (at least in my experience) is what makes it all worthwhile.

reliability is the inverse proportion to complexity

personal experience has the highest reliability (greatest MBTF) in a mechanically injected diesel motor with 378k on the clock, a 72 year old carbureted inline 6 in a truck with a 4-speed and 156k on the clock and carbureted 66 year old 2-stroke that starts on the 2nd pull every year. and the least reliability in new EFI motors, computer controlled transmissions and battery powered equipment. YMMV

 

[aspeck]

Your boat will run most efficiently at about 2/3s throttle ... enough to bring it onto plane, but flying across the water. So, you want a motor that will bring it to plane and hold it there without have to pile on the RPMs. If in fact your boat has a Coast Guard maximum HP rating of 150HP, you want to be somewhere in the 115-150HP range. The rest you will have to decide what you want and how much you want to spend.

 

[JimS123]

reliability is the inverse proportion to complexity

personal experience has the highest reliability (greatest MBTF) in a mechanically injected diesel motor with 378k on the clock, a 72 year old carbureted inline 6 in a truck with a 4-speed and 156k on the clock and carbureted 66 year old 2-stroke that starts on the 2nd pull every year. and the least reliability in new EFI motors, computer controlled transmissions and battery powered equipment. YMMV

OMG, I just replaced ALL my lawn equipment (except tractor) with battery powered tools.

 

[jimmbo]

How long are the Batteries good for before they lose Capacity, and what are the Replacement costs, and Disposal Protocols/Costs?

 

[Thirsty Endgrain]

Well there's a 150hp Yamaha for sale locally that is 100lbs over the weight of a 150hp 2 stroke. From 380lbs. to 480lbs.

The transom is new, but I still hesitate to go 100lbs over the rating of the weight that was thought of during the boat's design way back when.

Supply for used outboards is just so limited that it doesn't leave many options...

 

[jimmbo]

The 4 Strokes are certainly Porkers, and are a lot slower with the Holeshot. I can see why there are a lot of People asking if they can Ski with 115s and even larger engines, when with 2 strokes, a 40, even less would have pulled them up

 

[hardwater fisherman]

How long are the Batteries good for before they lose Capacity, and what are the Replacement costs, and Disposal Protocols/Costs?

Also many companies change the style of tools and batteries often. I have a pile of saws , drills, sanders etc. without batteries.

 

[jimmbo]

Also many companies change the style of tools and batteries often. I have a pile of saws , drills, sanders etc. without batteries.

Keeps the Shareholders Happy

 

[dingbat]

Well there's a 150hp Yamaha for sale locally that is 100lbs over the weight of a 150hp 2 stroke. From 380lbs. to 480lbs.

The transom is new, but I still hesitate to go 100lbs over the rating of the weight that was thought of during the boat's design way back then

The transom strength has absolutely nothing to do with a boat’s HP rating.

Maximum Boat Horsepower Calculator – Boating Hub

If you’re curious to know what the largest engine you can legally put on your boat is, try this calculator. It uses the federal regulations to compute the largest outboard engine you’re allowed to install. The federal government regulates the horsepower of outboard motors that can be installed...

towers4boats.com

 

[JimS123]

How long are the Batteries good for before they lose Capacity, and what are the Replacement costs, and Disposal Protocols/Costs?

I bought a 40V weedwacker 3 years ago. It gives me 2 full yard jobs on 1 charge. Recharge is 1 hour when dead. Liking it so much I bought a second one this year for my cabin. Then, a chain saw and a leaf blower. All 4 units use the same battery so recharging half way thru a job is never needed.

Their performance is superior to the 2-stroke gas model I had before. Plus, no gas cans, oil, etc.

A second battery costs about 1/3 as much as the original tool / battery / charger combo. So, yes very expensive.

I have a B&D 12 volt saw / drill / sander at camp. The batteries lasted about 10 years, and brand NOS batteries are available on Ebay. Very cheap now for obsolete tools.

In contrast, my 2-stroke equipment was noisy and forever had cabs plugging, requiring constant adjustment.

The convenience beats all. If they go dead, I'll simply buy a new one.