Best prop fuel economy

[Mrobi]

Hello all
I have a 1991 Larson 18 laser se
With a mercuiser 3.7L alpha 1 Gen I and I currently have a 17x14.5 prop. At WOT I'm running about 38mph at
5300 rpm. I am not familiar with the 3.7L. But 5300rpm seems a bit high (at least compared to the 3.0L , 4.3L and 5.7L that were in our previous boats which were in the low to mid 4000s) From the research of done I'm thinking a 19 or 21 pitch would get me closer to the results I'm looking for. Top speed is not all important to me. But me and the wife are mostly on the river and we enjoy taking 3 or 4 hour rides.(or as far a tank of gas will get us). Kids are grown so there will be little need for quick planing. But don't want to plow for a 1/4 mile either. Any advice would be appreciated. Thanks

 

[WesNewell]

Mercruiser 3.7 Specifications

Mercruiser is the inboard marine engine division of Mercury Marine. The typical Mercruiser engine is an inline 4-cylinder, V6 or V8 engine produced by General Motors and based on existing automotive engine designs. However, during the 1980s, Mercruiser designed and built an engine that was not a...

itstillruns.com

4000-4600 rpm, so unless you're pulling skiers or such, you you probably want a higher pitch prop.

 

[alldodge]

I think your tach is off, check it with a hand held

 

[Bikestdy]

Depending on which 3.7 you have your max engine speed could be different (see image). It would be best if you check the sticker on the top of the valve cover for what your max rpm range actually is.

I have a '85 4 Winns 170 Horizon (17ft) with the 3.7 (Merc 470). I run a 17p 4bl Turning point aluminum prop (I don't remember for sure but I think with this prop I am around 5000 RPM at WOT) (with a 17 3bl I was at 5000rpm, Turning point says no loss of RPMs with their 4bl props and I don't remember seeing a difference), but I use my boat primarly for water sports, for me a 19p 4bl (4000 RPM) puts me in the proper rpm range, but didn't have enough hole shot for water sports. For me to the difference between a 3 and a 4 blade prop on my boat was a night and day difference. It was my parents boat that I got from them (boat has been in the family for about 35yrs, the last 7 with me), when my dad drove it after putting a 4bl on it, he also was blown away at what a difference it made. It runs smoother, handles better, and pulls harder. I did not notice any difference in top speed (as Turning Point claims with their 4 blades). He said he would have put a 4bl on it long ago if he knew it would make such a huge difference.

In you case (as your boat is nearly identical) I think a 19 or a 21 would be what you need, depending what prop your using. If you use a basic prop probably a 21p or if you use a "performance aluminum prop"or stainless then the 19 I think will probably be right.

 

[Mrobi]

Thanks bikestdy. great to find someone with this engine. Thanks for the great prop advice. What is your cruising speed and rpms with your setup.? And do you have any idea on mpg or gph? Is your 3.7 a closed cooling system? Any info on your experience with a closed cooling system would be greatly appreciated. Dont wanna wear out my welcome but I will probably have a lot more questions about it once the weather allows us to get on the water?

 

[QBhoy]

I presume you have the prop numbers back to front and it’s a 17” pitch and not a 14.5” maybe ?
I think you need to get more pitch on that ASAP. Don’t use it again at those rpm I’d suggest. That engine is weak enough without stressing it even more.
Go easy on that engine I’d suggest and check that water pump for weep at the seal often. Check the coolant too. Once it fails there...they just never recover. It’s a never ending spiral of doom and expense for ever after. I feel that strongly about the horrors of that engine...I honestly urge you to sell it before it fails. Then buy a boat with a 3.0 or 4.3 in it that won’t let you and your family down. Because it will. There is no doubt about it.

 

[Mrobi]

Thanks QBhoy.. yes it's a 17 pitch.. from the little research I did on that motor I've came to the conclusion that since it's still on the water it must be a reliable one(knock on wood). It does have the 4" intercooler(if that's what it's called) instead of the 3". But thanks for.your concern.

 

[QBhoy]

Thanks QBhoy.. yes it's a 17 pitch.. from the little research I did on that motor I've came to the conclusion that since it's still on the water it must be a reliable one(knock on wood). It does have the 4" intercooler(if that's what it's called) instead of the 3". But thanks for.your concern.

No probs. Hate to be a mr negative. Apologies for that. Just had far too many ruined days caused by no less than 3 of these engines. Great when they are running well though.

 

[Scott06]

If your tach is accurate I would think a 19" would bring you down to 4900 or so, but defintiely verify the tach before investing in props....
17" is typically used as a pwoer prop for skiing or watersports, so 19 should be a good blend of holeshot and lower cruising rpm

 

[Mrobi]

Bikesdy
The sticker says 4200-4600 rpms. Thanks for pointing me to it.
Scott06. The seller told me that the boat. Belong to a family that was on the big side and enjoyed tubing(not sure exactly what that meant but I'm assuming the boat was loaded down). So I imagine your right. They needed the hole shot.
Once I get the tach check will make a decision on prop.

 

[Bikestdy]

Thanks bikestdy. great to find someone with this engine. Thanks for the great prop advice. What is your cruising speed and rpms with your setup.? And do you have any idea on mpg or gph? Is your 3.7 a closed cooling system? Any info on your experience with a closed cooling system would be greatly appreciated. Dont wanna wear out my welcome but I will probably have a lot more questions about it once the weather allows us to get on the water?

Your welcome, it isn't often to find someone here with a similar setup so I was kind of excited to see one that I had real experience with for as loosely as I follow the forums.

I really haven't paid any attention to what my cruising speed is, and I do not know what my MPG or GPH is (I don't have anything that measure it).

I do have the closed cooling system, but I haven't really had to do anything much with it. But feel free to ask your questions and I will do my best to answer.

I am really hoping to get my boat out again this year as last year it stayed in storage because we also camp and the season competes to do both, as well as the birth of another child mid summer (and my wife doesn't care for the boat much, but humors me because I love it and it was my childhood boat).

 

[nola mike]

C'mon over to the mercruiser forums. Many knowledgeable about these engines, although I think we're down to one fan of them (me).