Is bigger really better?

[witenite0560]

That's an attention grabbing title, eh! LOL I'm looking for some opinions and since this is engine related I may have posted this in the wrong place. If so, I apologize. But, it's also quite a broad question related to boats, so not sure where it goes.

Anyway, I know there's lots of opinions out there on engine size for a particular boat and I know there's a lot of variables. But, please give me your thoughts:

I've got a 16', 1968, Crestliner del rio. Great solid little boat. I bought it last summer. It came with a 1970 115hp Evinrude. I've noticed a lot of boats the same size with much smaller engines. Also, I've noticed the data plate on the boat says the Max HP rating is 100hp. I'm thinking she's probably way over powered and I'm burning a lot more fuel than I should be. I use the boat primarily just for fishing here in Idaho. Normally, just me and maybe a friend on board. Might use it to pull a tube or something now and then. I've been looking for a good motor in the 50hp - 70 hp range for better economy, less weight, etc. Am I on the right track? What size motor would you recommend? Thanks.

 

[dirtyoldman]

Re: Is bigger really better?

If the transom's solid - I say it's not broke - don't fix it.

Can you adjust the throttle linkage to limit the carburetor from opening all the way? I doubt it's affecting economy at all, and you would be sacrificing an engine that you say is solid for one of unknown condition.

 

[HAV2FISH]

Re: Is bigger really better?

I would stay with the 115hp. The extra 5hp wont make a difference as far as the transom goes. You don't always have to use the extra power you have with the 115hp over a 75hp etc. but it is nice to have if you do need it. The fuel economy from one to the other will save you a few dollars but how long will it take to offset the cost of another motor.

 

[Home Cookin']

Re: Is bigger really better?

I wouldn't replace it unless it dies, but when it does, keep this in mind: you have OMC controls so you will need new controls if you switch brands. Old OMC's are the best but newer ones aren't. If you are in fresh water you get along life out of the motor; in salt, 20 years is old old old.

I am looking at these dynamics now, with a dead 70 hp Johnson, 20 years old on a 20 year old boat.

 

[witenite0560]

Re: Is bigger really better?

Yep, the 115 runs pretty good. Seems to get better the more I tinker with it. The former owners apparently were satisfied with only forward gear. I replaced the shift button switch assembly and it works like a charm. Made sure it had the right oil in the lower unit, too. Part of what made me think a smaller engine might be a good idea is the motor was pretty carboned up. Since outboards are somewhat designed to run best at fairly high rpm I thought perhaps running it at lower relative rpm might be contributing to the carbon build up and this might not be as much a factor with a smaller motor running at higher rpm.
Bubba, thanks for the advise, I think my next one is going to be a small cuddy cabin. But, not just yet, I'm still having fun making this one better.
Cookin', Johnson is in my blood, really it's my last name, I couldn't go with anything but OMC!! LOL

 

[The Famous Grouse]

Re: Is bigger really better?

Outboards build carbon frist and foremost because of bad fuel and bad oil. Use high quality fuel and oil, and carbon buildup is vastly reduced.

Beyond that, the only way to answer your question about economy for sure is to get a fuel flow meter. Installed, it would cost roughly ten times what your outboard is worth.

Why mess with it? Any difference in economy is going to be minor and the hassle and cost of swapping out engines would be difficult to justify unless you're racking up hundreds of hours a year.

If your question was should you replace a setup with twin 200 HP two-strokes on a big offshore rig, then there's be cause for serious consideration because the fuel savings could amount to thousands of dollars per year. But for a small recreational boat with a single outboard I can't see where it would be worth going from one two stroke to another hoping to save a few pennies.

Grouse

 

[Nandy]

Re: Is bigger really better?

You got real good advise here. One other thing to mention. I have a 70 1975 and a 115 1974 evinrude motors. I just swapped the 70 for the 115 about a week ago or so. The difference in weight is around 46 pounds. The 70 gives me great mpg (4.8 if I recall right) and I have not calculated the 115 yet but I can tell is way more. 115 cruises at 3k to 3.2 k rpm, the 70 does at 3.8k rpm (cruising speed for both is 20mph). As you can see, the 115 does the same work as the 70 with less effort but it is yet uncertain to me what is the mpg difference at cruising speed. I do love the extra power punch of the 115.
If I were you I would leave your rig as it is, which seems to be what you are planing anyway. The only worry is being overpowered in case of an accident and someone trying to make a case out of that.

 

[windsors03cobra]

Re: Is bigger really better?

In most cases the bigger the motor that better.

 

[JB]

Re: Is bigger really better?

In most cases an outboard boat will give the best combination of economy, handling and performance with about 70% of the rated max HP.

That means that, yes, you are overpowered, abusing your hull and burning more fuel than needed to move your boat around.

The idea that a 50HP running at WOT burns more fuel than a 100HP running at half throttle is pure nonsense.

You will do a lot better with a good JohnnyRude 70 looper.

 

[45Auto]

Re: Is bigger really better?

From JB:

The idea that a 50HP running at WOT burns more fuel than a 100HP running at half throttle is pure nonsense.

I don't suppose you have any real data to back that statement up, do you? It's very hard to find real testing of the same hull with different motors. I've seen some tests on different I/O's in the same hull, never seen any direct comparisons of outboards. I can see no reason that OB results would show any different trends than I/O's.

I'm always interested in actual test data (not somebody's BS opinion, which is pure nonsense) that let's me learn more about boat performance. From what I've seen, fuel usage is determined by boat weight and drag. Engine size appears to be a minor factor so long as you are talking about equal speeds in identical hulls. A larger engine working at lower RPM can easily be more efficient than a smaller one working at higher RPM.

Boattest.com has lots of boat tests. They had an identical Sea Ray hull (2006 185 Sport) with a 135HP 3.0L, and also a 190HP 4.3L version. Their results are below:

3.0, 135HP:

4.3, 190HP:

Here's a graph of the relevent data above (MPG vs Speed) of both engines:

As you can see, once you're on plane in the low 20's, the 4.3L is delivering better fuel economy. And if you want to use it, it also has the extra power available to push the boat to 50 MPH vs the 39 MPH of the 135HP.

If you have any data on outboards in equivelent boats, I would be very interested in seeing it.

 

[witenite0560]

Re: Is bigger really better?

Thanks, these are just the kind of opinions I'm looking for. Keep'm coming, please. Thanks.

 

[Bifflefan]

Re: Is bigger really better?

Thanks for the great info 45auto. i too like to see the numbers and not so much opinion.
Very interesting reading.

 

[mphy98]

Re: Is bigger really better?

sorry i just can't resist, been looking at this for three days. on the title, you would have to ask my wife!

 

[witenite0560]

Re: Is bigger really better?

I'll refrain from asking your first mate whether the dimensions of your motor are adequate. However, your comment just goes to prove that it is an effective title!! LOL