Ideal engine size.

[arko]

Hi. I have a Plancraft Sabre MK3 17ft built around 1985/86. It came with a Mariner 2 stroke 75hp outboard. I have been reading that this beauty guzzles a load of fuel. I would ideally like to race around the bay at throttle for more than 10 minutes and also like to slow-boat at times. Ideally the boat should be on the water for around 2 hours at a time. What would be the best recommended size motor for this situation?

 

[MikDee]

The One you got!

If you want to go switching motors, smaller is better, but not necessarily too small. Check the Coast Guard rating plate on your boat for maximum recommended horsepower.

 

[achris]

No, smaller is NOT better. Buy as BIG as your budget and the rating of the boat will take. There is a myth that bigger engines use more fuel. Complete rubbish. If you run at the same speed, the bigger engine will use LESS fuel as it is not working as hard (proportionally).

Cases in point. 1. My boat had an Mercruiser 165, inline 6 when I bought it. Later I replaced it with a Mercruiser 4.3LX, V6. About 45hp more, and it used LESS fuel, by about 20% for the same boat speed. 2. My car (Landcruiser 100 series) original engine was a 4.2litre straight 6 diesel. I replaced that with a 6.5 litre Turbo V8, and it now uses less fuel towing the boat than the original did without towing...

You have also been reading incorrect information. Those engines are quite good on fuel, when compared to other brands of similar size. No boat engine is even going to be as 'economical' as a car, the loads are just that different. My car and boat have around the same power, the car a little more. The boat uses around 1.3 litres per nautical mile (about 4 miles per gallon), the car (cruising not towing) around 20 miles per gallon.

Chris.......

 

[MikDee]

After re-thinking this, I tend to agree with Chris!
Driving a boat is a lot like driving a car uphill, All the time, (there is no coasting on the water) Hence the most power you have, the easier it is to do with a bigger engine.

 

[TD_Maker]

The Mariner/Mercury 4cyl 2 Stroke is like any other outboard really. The 75 will suck up the fuel at full throttle and 5500 RPM. However, back out of the throttle a little, and the motor will be much more economical. My motor was much better at the 3500-4000 RPM range. Get on top of the water and back it off a little...you will be surprised.

 

[metriccrescentwrench]

Hence the most power you have, the easier it is to do with a bigger engine.

That's just an old-wives tale. To go a certain speed in your boat (30 mph for instance) requires the exact same amount of power no matter what size engine you have hanging on the back. So say that a boat will go 30 with a 50 hp engine at WOT, that means it takes 50 hp to go 30 in that particular boat. Hang a 90 hp engine on that boat and it's still gonna' take 50 hp to go 30 mph, it's just that now you have some extra power so you can go faster than 30 if you choose.

And guess what.... most engines will burn pretty close to the same amount of fuel to put out a given amount of power, meaning that to go 30 mph in this example is gonna' take about the same fuel flow rate with either engine. There will be some variation when you're talking about vastly different sized engines due to there being more internal friction in the larger engine, or when you're comparing newer technology to old (DFI vs. carb), but not a lot. And of course a racing engine is gonna' suck fuel no matter what, but not many of us will be dealing with that.

 

[achris]

So your saying if the boat needs 60hp for a given speed and that's the speed you'd like to travel at, then get a 60hp and run it 100% the whole time... :facepalm:

 

[Silvertip]

Boats have a throttle lever. It does not need to be pushed full forward all the time (it's not an on/off switch). Determine what the maximum HP rating is for your boat (there should be a plate near the helm with that data on it). When selecting an engine for that boat you need one with about 75% or more of the maximum rated HP for that boat. If the boat is rated for 100 HP then it should have a 75 - 100 HP engine. Any less HP and you will not pull skiers/tubes etc., It will also be a slug and likely burn more fuel than a bigger engine. If fuel consumption is a major factor for you then you need to find a different activity as is so often pointed out. Boats are not noted for economy -- no comparison to cars.

 

[JesterGrin_1]

Wow There are many factors that effect fuel consumption. I know a pretty good amount about cars and from in depth reading a little bit about boats. But in reality fuel consumption is pretty universal. They may seem different but are really the same.

For a Boat we have what is called the Hull and the design of he Hull has a speed of which they are designed to run in efficiently. If one tries to go above the designed Hull speed then they will use a bunch more fuel to do so with little return in speed.

And as said a Larger Engine can use Less fuel if the boat is run at the most efficient Hull speed with the correct prop chosen to do so at the engines sweet spot for RPM.

But each Boat and Hull even if they are made by the same manufacturer in the same year will be a little different but should not be much but again each Hull is a law unto itself.

I would also look into obvious things such as how smooth is the Hull. As in how good is the Paint and is it waxed well and taken care of. A smoother Hull is a Smoother running boat and takes less HP to push it through the water.

A rule of thumb I usually go by is to get a boat just above a smooth plane and keep the speed there. And it usually works pretty well.