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I've followed the recent discussions about repowering with interest. Often the MPH is a central issue, but I'd like to tap the experience of others around here in regard to range, which is another way of saying MPG. My little 16 has a limited capacity to carry a lot of gas, and I was wondering if maybe a 70 HP or so wouldn't be a better choice than a 90 or 100. I'd gladly trade top speed for fuel economy... to a point. Also, any tips about fuel economy would be welcome!

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I can tell you that the 60 hp ETEC that I have sips gas and oil. It has actually blown my mind as to how good it is on fuel economy. It also hauls butt too! :D

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Dan,

There has been lots of theory's and discussion but nothing scientific that I know of...

So, lets look at this:
Let's say a persons boat always weighs the same. In your case, lets just say 2500 lbs.
You always take out the same amount of people every time out so the weight does NOT change...

Example 1 has a 70 hp on the back and you cruise at 30 mph....

Example 2 has a 90 hp on the back and you cruise at 30 mph....

The 70 hp is going to have to work harder to keep the boat up to 30 mph that the boat with the 90 hp. The 90 hp boat will be loafing along while the 70 hp boat is working...
Which boat will have the better fuel economy?

Again, nothing scientific that I can find but I think the fuel consumption would be fairly close...

The main advantage of the 90 hp is the engine will probably last many years longer because it would not have to work as hard as the 70 hp engine... Plus, if you ever needed the extra speed, you would have it with the 90 hp.....

This is assuming that both engines are DFI and other factors being mostly the same..

With ioptfm, his boat is rated for 70 hp and a new 60 hp is only 10 hp from the max..

For your boat, 100 hp is the max so dropping down to a 70 hp is 30 hp under the max.
I used to have a 70 hp on my Montauk.. Wide open was 30 mph... I did run the engine at almost wide open throttle when traveling and I never had any problems with the engine, but a 90 on the back would have been loafing along....

I have never been one for top speed myself. Economy and longevity is my goal.
I'm thinking about repowering my Outrage 18 with a 115 hp E-Tec... That's 35 hp under the max but then the engine is $3k less than the 150 hp.... The new E-Tec will get much better fuel economy than the 150 Evinrude I have now, guaranteed... I have also been on an Outrage 18' with a 115 hp on it and the boat did 40 mph... Mine only does about 42 to 43 with the old 150...
I haven't totally made my mind up yet, but that is my thinking.
Evinrude was going to make the 130 hp E-Tec.. but I think they changed their mind or put it on the back burner... If they did, I would probably go for the 130 hp....

I don't think you will find that much fuel savings (if any) with a 70 hp versus a 90 hp if you choose one of the newer engines produced today...

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Joe, you logic is exactly why I went with the 60hp vs the 50 hp. I am not one that is all that big on speed, but I didn't want to have to be at higher RPMs the majority of the time. The same goes for the theory that you shoudn't buy a used motor that was on a pontoon boat because they run at top rpms the majority of the time.

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Short answer!

Most outboards, IMHO, will get the best MPG cruising (normal load) at about 4K RPM. So I would, and do, match an engine to the boat that will do so.

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RGT,

That's a good point. I should have mentioned RPM instead of, or along with, MPH....

My boat and motor combination likes to cruise at 3600 or 3700 RPM right about 30 mph..

Every boat and motor combination has a sweet spot. You can actually hear it when the engine doesn't seem to be working real hard and she's just cruising along. RPM and MPH will vary for that sweet spot, but should be somewhere in the range from 25 to 30 MPH and 3500 to 4000 RPM... unless of course the boat is underpowered....

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Points taken. Good education. Thank you!

ioptfm: nice boat... great job on the woodwork!

I should be looking for a 90 or 100. After all, my Yamaha 115 might work well after all... but I'd need to get a power trim unit for it (it's missing). I'm concerned that weight the weight of the 115 two stroke may require me to put a lot of weight forward to balance the boat.... anyone know what an '86 Yamaha 115 two stroke weighs? How does it compare to 4 stroke 90s and 100s?

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Dan,

I have an article for the larger, 18' Whalers, that includes the 115 hp motors.
http://www.whalercentral.com/readarti...ticle_id=6

The new Yamaha 115 hp 2 stroke weighs in at 358 lbs.
I don't think they changed the 115 hp that much from the 1980's....
I think you should be good to go with that 115 hp as long as kids are NOT going to operate the boat.

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Makes me feel better. I've heard that the Yamahas are very durable. A buddy of mine said that if you visit the little fishing towns in Mexico, they are loaded with those mid-80s Yamahas --- work boats, fishing boats, and they don't ever seem to work on them! And an extra 15hp under my belt won't hurt if I have to scoot in to avoid some weather.

I checked both your articles, Joe, and see that the weight is not as bad as I imagined it could have been.

Three police whalers I ran across last weekend in Delaware City:
[IMG]http://i204.photobucket.com/albums/bb77/DelawareDan/police%20Whalers/policeWhalers2.jpg[/IMG]

[IMG]http://i204.photobucket.com/albums/bb77/DelawareDan/police%20Whalers/policeWhalers.jpg[/IMG]

[IMG]http://i204.photobucket.com/albums/bb77/DelawareDan/police%20Whalers/policeWhaler1.jpg[/IMG]

B)

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Dan,

Yes, this is a very interesting topic particularly as Gas goes above 3 bucks a gallon for reg and much higher in many states... I down sized partially for this reason from a V6 merc to a Yamaha 90 HP. My Super Sport Limited is the same hull as Montauk, and is begging for a 115HP, but the plate says 100 HP....:|

Generally speaking there is a saving when dropping HP if that is waht you really want to do. But if you need to run your outboard at higher RPMs as Joe points out a few post above. Could be a wash. Here is an example, with my V6 Merc 175 HP; I would pull a water skier at 32/33MPH at 3200/3400 RPM. With the 90 HP, it needed another 1000 RPMs to keep a similar speed. Note that there are many other variables here. The V6 burns about 8 per gallons per hour, the 90 burns 7 Gals per hour, maybe a drop less… I was very surprised when I read this, not much savings. Most of my boating is at much slower RPM., therefore I will burn much less fuel. In that scenario, V6 Merc will burn about 3.5 GPH, at 1500 rpm, the 90 1.8 gph, about half. Speed is not an issue when cruising around with the wife.

The same kind of thing seems to hold true on a smaller scale 90HP vs. 70 HP below is a link to Yamaha performance data on many different outboard combos. I suspect this will also be some what generally representative for others brands as well 2/4 stroke and DFI… Since I do can't locate Merc data my comparison with the V6 uses a Yamaha carbed 150 HP 2 stroke it’s newer technology and may be better on fuel. Looking at the data quickly 70/90 HP does seem to be a saving about 15% at same speed. Check this link out, use the pull downs to gather the data you are looking for...

http://www.yamaha-motor.com/outboard/products/perfbull.aspx?ls=outboard

The ETec claims to be the best on fuel overall... Just another thought on range, at roughly 5 MPG for a 70/90 HP, and optimal speed 3500 RPM for best MPH, and a 28 gallon tank which should fit under you drivers seat. Your range could be about 60/70 miles…. That is a long long way out in the in to the Atlantic?

Happy Father day to all, I will be boating…B)

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Many of you have seen this stuff before, but I'm fascinated. :o Compared two Yamaha motors, both 2 stroke: the 90 and the 115, both on Carolina Skiffs, very similar to BW.

The 90 was on an 18', 800 lbs (1225 as tested); the 115 was on a 22'8" at 1200 lbs (2250 as tested)... much heavier boat, but still, the data are interesting.

Sweet spot for the 90 was 5.36mpg at 4000rpm, 29mph
top speed dropped to 4.58mpg at 5550rpm, 41mph

Sweet spot for the 115 was 4.75mpg at 4000rpm, 29.9mph
top speed dropped to 3.09mpg at 5600rpm, 41mph

Wish they would have tested that 115 on the 18 footer!

There is a significant drop in MPG between 4000rpm and 5500rpm.... might be called the "bitter spot"! It actually gets better again, as you approach 5500.

As Joe pointed out, all things being equal, the bigger motor might be a wash, at least between the 90 and the 115, and it won't work as hard, thereby lasting longer.

Enjoy your boating tomorrow, and thanks for the info. (I'll be enjoying working on the boat!)