Up powering a boat?

[Catching Nemo]

Not sure how to phrase this correctly but hopefully you get my drift.

Is there any general rule of thumb for how much hp is lost with additional weight gain? For instance, you are currently running a 130 HP outboard prop and you go to a 225 HP but also add on 100 pounds in engine weight. What is the net increase in horsepower? It would be 95 if both engines weighed the same but what would it be with a heavier engine (100 pounds or 150 additional pounds)?

Bottom line, are you better off putting a 175 HP engine on if it weighs the same as the existing engine or moving up to a 200-225 with anywhere from a 100 to 150 pound increase in weight?

Hopefully that made some sense :shrug:.

[R Jackson]

If your boat is rated for up to 225hp and you find 130hp lacking, then step up to the bigger motor. Lets say you weigh 130lbs and can do 10 pull ups. You decide to hit the gym and bulk up to 225lbs...all muscle. Now you can do 75 pull ups even though you weigh 95lbs more. Does that help?

[Catching Nemo]

Sort of, I was looking to see if there would be a balance of weight over hp. For instance, can I get the same performance increase over my 130 from a 175 instead of a 200 or 225 if that engine weighs a 100 to 150 pounds less than the 225 (with all other things being equal)?

My current engine weighs 505 lbs, has 130HP, and the boat tops out about 35MPH (over land) with 2 people and gear. A new 4 stroke 200 or 225 would be about 588 lbs to 640 lbs (depending on the model). However, there are some 2 stroke engines (Merc Optimax and Yammie HDPI that weigh 450 lbs for a 175HP and about 500 lbs for the 200-225 series and they are less expensive.

I was told that by getting a 200-225 4 stroke, the additional weight would basically equal another person and therefore would reduce the perfomance to a lesser HP engine. If I go with a 175 Optimax that is 70 pounds lighter than my current engine, it would provide similar performance to the 200-225 because of the savings in weight.

If the answer is yes, and the 175 will do that, then the next question will be is it worth it to go to the a 2 stroke over a 4 stroke? I guess the real easy answer is just to get a 2 stroke 225 for the same weight as my 130 and call it good......

In summation....aaaaarrrgghhhh.

Thank you

[R Jackson]

I'm not sure you can look at as just adding another person. The motor is providing more hp too, not just dead weight. It's not like it's 100lbs of ice, that would be dead weight. I would think that you really notice the up to 50 extra hp. Also, consider how much you are spending to get 45hp more vs how much would you be spending to get 95hp more.

[Pickles]

One other thing to consider is that if you go with a DFI type two stroke, most require the use of special synthetic 2 stroke injection oil. I believe that synthetic 2 stroke oil runs about $6.00 a quart. At 1 quart for every 12 gallons of fuel, it will cost you an additonal 50 cents per gallon of fuel burned for your 2 cycle synthetic.

[SouthCoastStu]

I assume your talking about a planing hull. It seems to me that once you're up on a plane, the extra weight would not be that much of a factor (friction loss on the hull will be about equal). I would be more concerned about how the hull would sit in the water when trolling (might be kind of stern heavy). Other than that I think it's just a matter of how much money you want to spend and how fast you need to go. Of course, I'd never exceed the hull manufacturers recommendation. I'd only go with a 2-stroke if I was running a jet, but that's just me.

[Rescue3]

Yep...synthetic oil can be very pricy. I just ordered 55 gallons of XD-100 (Evinrude's brand) ... don't even ask :hoboy:! It's running about $20 a gallon in bulk prices so do the math :shrug:

The upside is that the new DI engines use much less than the older 2-strokes so the 1/12 figure might not work. I know that 120 hours on a new 90 HP E-TEC used just 2 1/2 - 3 quarts of the XD-100. Also, at least on the E-TEC, you can run TW3 so long as it's not in cold weather. On the new pair of 225's, I've been told to expect to use 6-8 gallons each for the season, and that's 1,200 hours of run time.

[Tinman]

A rule of thumb is that it takes 1 hp to move 30 pounds of weight at 30 mph. In other words, a 100 pound increase in motor weight "uses up" about 3.3 horsepower just to move the extra 100 pounds. I got this rule of thumb from Jim Martenhoff's book, "The Powerboat Handbook", which is long out of print.

However, also consider the effect on boat trim. Extra weight on the transom can cause the stern to sit low which is unsightly and dangerous.