Prop advice on old Tri-hull 3.0 mercruiser.

[jtuffy]

I want to replace the prop on my 1973 17' Renken Tri-hull bow rider. The motor is a 140 hp mercruiser and an alpha one sterndrive (or actually may be pre-alpha I believe-Top eye hook model-). The boat is used mainly for cruising, and pulling skiers and tubers. The boat weighs about 1410 lbs and we usually have about 1400lbs of people and gear. I currently have a 15" pitched prop on the boat.

Currently, the boat currently has a good hole shot, planes fairly well, and does not have any trouble pulling tubers and skiers. However, when just cruising it seems that I have excessive RPM's if I run WOT. Additionally, my actual RPM's cannot be completely determined by my lack of a working tach or speedo. Therefore, we never run it wide open since it only excessively increases the RPM's without actually gaining speed. When running the boat we actually back off of the throttle about 1/5 from WOT when cruising, and the boat will stay on plan at about 2/3 throttle.

Question: I am thinking about moving up to a 19" prop. Is that to high for my boat? Will my boat become a dog out of the hole, or not be able to pull tubers /skiers? I do not want to sacrifice pulling for a little speed, but the WOT RPMs seem to be excessive. Any suggestions?

 

[billy4hp]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

I want to replace the prop on my 1973 17' Renken Tri-hull bow rider. The motor is a 140 hp mercruiser and an alpha one sterndrive (or actually may be pre-alpha I believe-Top eye hook model-). The boat is used mainly for cruising, and pulling skiers and tubers. The boat weighs about 1410 lbs and we usually have about 1400lbs of people and gear. I currently have a 15" pitched prop on the boat.

Currently, the boat currently has a good hole shot, planes fairly well, and does not have any trouble pulling tubers and skiers. However, when just cruising it seems that I have excessive RPM's if I run WOT. Additionally, my actual RPM's cannot be completely determined by my lack of a working tach or speedo, therefore, we never run it wide open since it only excessively increases the RPM's without actually gaining speed. When running the boat we actually back off of the throttle about 1/5 from WOT when cruising, and the boat will stay on plan at about 2/3 throttle.

Question: I am thinking about moving up to a 19" prop. Is that to high for my boat? Will my boat become a dog out of the hole, or not be able to pull tubers /skiers? I do not want to sacrifice pulling for a little speed, but the WOT RPMs seem to be excessive. Any suggestions?

It will help the pro's that post in this forum if you read the stickies at the top of the forum about how to get the best answers to your questions, etc...

You have given a real informative post but they will really need to know your WOT RPM and outdrive gear ratio which can be somewhat of a hassle to find out but is really important in order to give you a recomendation on what prop to go with...

Good luck...

 

[jtuffy]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

Sorry. The gear ratio is 1.98:1 And when I used the prop selector at Michigan wheel it suggested a 18" and WOT range 4200-4500 on my 3.0L inline 4 cyl.

 

[steelespike]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

While it appears a 15 isn't enough prop you really shouldn't be changing props without a working tach.Even if you select a prop from The best prop picker you need to be able to check the result.
Now way to know what rpm a 19 will get you but with your typical load it most likely will turn your boat into a dog compared to the 15.

 

[hwsiii]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

J, Steele is 100% right without knowing what your RPM are it is hard to calculate the correct pitch prop you need. If times are hard and buying a tach is a problem then let me know and I will get some information from you and figure out what you need. If times are not hard, buy a tach as all motors need one.


H

 

[fishrdan]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

I had a 17 pitch on my 73 16.5' SeaSwirl tri-hull and I liked it. Normal load was most likely close to yours since I had 4 batteries, trolling motor, anchors, chains, camping gear, 100# ice, etc. loaded on the boat all the time, oh yeah the foam was soaked too Only time I dropped to a 15 pitch was for high altitude.

My boat was most likely under propped, but I never pushed it hard, cruised at 3300, tops I ran it was 4000.

Oh yeah, have you weighed your boat? I was shocked how much my old boat weighed when I scaled it.

 

[jtuffy]

Re: Prop advice on old Tri-hull 3.0 mercruiser.

Everyone, including myself, has talked me out of a 19" I just ordered a 17", which is somewhere in the middle. I want to give it a shot. I will keep everyone posted, but I thought to myself..."Hmm... I don't need top speed, but little more would be nice, and, what the heck, I can always go back to my 15 and use the 17" for a spare if I have to.

Thank you to everyone for the advice! This is an excellent board that is really helpful to many new boaters!

jtuffy