Need the right Prop

[Dan73]

Hey guys,

I've got a 1962 20ft timber 'Quilkey' built ski boat.
The boat is running an inboard 350 chev crate motor and an Argo v drive w/dog clutch.
It runs beautifully however the V drive seems to rev out at about 4200rpm...

Im not sure of what the current set up is but has anyone got any idea of what prop I need to get the revs down but keep the power/speed up..

 

[Silvertip]

Re: Need the right Prop

Let's see here -- you want to "get the revs down" -- but at the same time you want to get the "speed up?" First, the engine is already lugging at 4200. Next, the outdrive isn't reving at 4200, the engine is. You have no idea what prop you have but knowing so little, we can only tell you that if EVERYTHING else is up to snuff (engine tuned properly, boat is not full of water soaked foam, isn't terribly overloaded, etc) then the prop you have has too much pitch so you would drop a couple inches to get the engine speed in the 4600 - 4800 rpm bracket. Speed is determined by engine rpm, prop pitch, and gearing. You can't have it both ways. Adding pitch "theoretically" would add speed but it takes HP to twist more pitch. Use a prop that puts the WOT rpm at or very near the top of the engine manufacturers RPM band with an average load the speed will be whatever that provides. You might -- and I say might -- be able coax 1-2 MPH more by overpropping but you lose hole shot in the process. You also my find it takes more fuel since the throttle must be open further than if the engine wasn't lugging. So -- prop it right and speed will be whatever that prop provides. There is no free lunch in this regard. As for the 350 crate engine, that could mean a lot of things. The engine you have may have a high performance cam which is not acceptable in a boat. You want a truck engine or a marine crate engine.

 

[Robbabob]

Re: Need the right Prop

A Sticky at the top of this prop forum states:

Read This First!!
ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

As silvertip points out, your information is somewhat confusing. Might even sound like you are cavitating when you say you "rev out", assuming you don't gain any more apeed when you increase rpm above the 4200.

Provide more information and get better information.

Good luck!

 

[Robbabob]

Re: Need the right Prop

By the way, welcome to iBoats!

 

[Faztbullet]

Re: Need the right Prop

Is it a marine or automotive 350 crate motor?The reason I ask as the 350 crate is offered as a cheapo(GM Goodwrench 350) and only makes about 200hp @5000rpm vs the marine version which makes 270-315 hp @4800 rpm depending on version