New prop question on 21ft Carver

[Tagerman]

I'm replacing my 15 x 17p prop tomorrow and I'm driving to a local prop shop. I had my heart set on getting a four blade prop (even though the diameter is a half inch smaller) to help me planing off when I have a few people on the boat. He says the boat doesn't have enough HP for a 4 blade to help me and I should stick with the three blade for planing out. The boat is a 1990 with a 5.7l Mercruiser (with thunderbolt ignition if that makes a difference) with an alpha one out drive and runs about 4100 rpm wot. I'm happy with my top speed and everything once I'm planed but getting there is a nightmare even with tabs. My current prop is ragged but I'm worried it won't make the difference I'm looking for. Should I go with the 4 blade or stay with a 3?

 

[steelespike]

Re: New prop question on 21ft Carver

Is the present prop a basic aluminum 3 blade not a ss prop with a lot of rake or other refinements?
Diameter is a secondary consideration. Its normal for a 4 blade to have reduced diameter. This is done to help maintain
rpm.

 

[steelespike]

Re: New prop question on 21ft Carver

It's worrisome that a 260 hp with a 17" prop struggles so to make plane and can't get into the minimum wot rpm range.
Is the motor in excellent tune? Is the bottom clean and smooth?What is the speed at 4100?
Do you know the gear ratio.Any water in the bilge?
If your sure the motor,bottom and the bilge are up to par;
you need to try to get the rpm up while its extreme a 15" 4 blade might help you get up on plane
and at least maintain rpm.You will lose some top end.
I do still think your problem is in the motor or area other than the prop.

 

[Tagerman]

Re: New prop question on 21ft Carver

The prop on there is in terrible shape it might even be bent a little. The motor is in tip top shape, I'm a mechanic I replaced plugs wires adjusted the timing and put a new distributer. Speedometer is broken so I'm not positive on speed. The bottom needs to be repainted so it's not perfect which could be slowing it down. is 4100 low? the tach could be little off it is an old boat but the motor is really rolling at 4100 I wouldn't want to run it much harder than that ever. The current prop is the original 3 blade aluminum prop. The motor seems to have plenty of power I just feel like the prop is kicking more water around than it is pushing the boat forward. With me and my wife only this boat jumps on plane no problem but with 6 people on it and a headwind it takes a little bit. Once on plane I can smoke little runabouts. The boat weighs 3900 lbs dry so its a heavy boat. My professional opinion is that my problem is in the prop (and partially in the worn out bottom paint but I'm not doing to repaint until the end of the season). Also an update I found out one of my trim tabs is leaking fluid from the hose so this was most likely keeping them from operating properly. My big question is 3 blade or 4 blade prop?

 

[steelespike]

Re: New prop question on 21ft Carver

Low end of wot rpm is 4200.
What is the speed at 4100?Maybe borrow a gps or some cells have a gps app.
Depending on your speed at 4100
A 4 blade may be in order.
Maybe borrow a shop tach or have a tech ride with you to prove the rpm.
If we can't prove your rpm I would stick with a 3 blade 17 until everything is ironed out.

 

[Tagerman]

Re: New prop question on 21ft Carver

I appreciate the input SS I'm going to purchase a 3 blade along with having mine rebuilt so I have an extra. I'll take my shop tach out this weekend should work on a boat like it does on a car and I'll come back with accurate data. My tach doesn't seem to stick but I noticed with everything off it says 500 rpm so its probably off.

 

[Tagerman]

Re: New prop question on 21ft Carver

Update! Went to Dan's Prop Shop here in town and he was extremely knowledgeable especially since he had a similarly powered and sized boat. His recommendation was a 17p 4 blade stainless he said that is what he had on his boat and it works wonders on a boat the size of mine. I went with an aluminum 17p 15" diameter 3 blade prop. After all the discussion I showed him a picture of my old prop and he was blown away that I could even get the boat to plane with that prop. He said keeping my use of cruising to swimming holes, dropping anchor and grilling he said going with the OE prop would probably be best for me and sold me a brand new Twister brand aluminum prop plus the hub kit for $100 even. I'll let you know how it goes HUGE storms kept me from installing it today. Thanks for all the help SS. AND to anyone in the Chattanooga, TN area Dan's Prop Shop is a little hole in the wall full of knowledge and Dan is a super friendly guy. He is even going to rebuild my old prop so I have an extra!