Selecting the correct prop

[justinteoc]

I just purchased a new (to me Boat), it's a 2000 Four Winns Horizon 170 with the 4.3L engine with the SX-M outdrive. It was owned by a doctor with plenty funds so it was very well maintained, I noticed on the info plate on the engine WOT should be 4500 Rpms, it will run 51MPH at 5000 Rpms with a 14.3X21 prop! The question is should I change the prop size to lower the Rpms a little or just back off the throttle a little? If I change the prop would I get more top speed or less? I never run the boat that hard I just had to test it out , I like to cruise between 3500-4000 Rpms to keep from overstressing anything.. I'm happy with

 

[Maclin]

If you are sure the tach is reading accurately then the 5000 rpm is too much over the top range in my opinion and you are right to be thinking about it.

Some depends on what your type of boating will be. For instance, if you are thinking about skiing or tubing with passengers then I would stay with what you have and just watch the throttle when not towing and/or fewer passengers. If it is just, say, you and the Admiral cruising then I think you need to try a 23 pitch to be safer regarding potential over-revving.

If all the numbers are accurate then you could probably still over-rev with a 23, but not by much and watching the throttle would work for you.

Your "new" speed would be about the same, maybe +1mph or so, just at lower (and safer) RPM.

 

[justinteoc]

Thank you, the current prop needs some attention anyways,( edges are a little rough) so I'll try a 23 pitch and if for some reason I'm not happy I'll send the current one off for balancing!

 

[Sea Rider]

That's the issue boating with a fixed load say, + 2 up on a constant basis or with multiple boaters, pulling water toys heavily loaded eventually. If boat's load will be varied frequently I would stay with current prop and just throttle bit less watching tach when lightly loaded. Can cruise at whichever rpm/speed you like most.

Happy Boating

 

[jimmbo]

I just purchased a new (to me Boat), it's a 2000 Four Winns Horizon 170 with the 4.3L engine I noticed on the info plate on the engine WOT should be 4500 Rpms, it will run 51MPH at 5000 Rpms with a 14.3X21 prop!

Your engine has an Full Throttle rpm range. Examples for Volvo were 4200 - 4600, 4400 - 4800, 4600 - 5000, and 4800 - 5200. Newer motors may even be higher. Most carbed Volvos from around 2000 were in the 4400 - 4800 category. You need to know/find out what that range is. Trying to prop it for exactly 4500 rpm may end up being an exercise in futility.

Under propping then throttling back to keep the revs down isn't without risk. The ignition advance and fuel delivery curves are based on certain engine load/rpm ranges. If under propped and engine is throttled back to keep the rpms down, the fuel delivery system, carb or FI, could get higher than normal vacuum signals which may result in leaner than required fuel mixtures than needed

 

[justinteoc]

It's a 2000 model which is carbureted, it also says on the data plate 4400-4800 now that I take a closer look.. Thanks for the info, The guy I purchased it from said it had a 14.3x23 size from the factory and he always carried a full load so someone recommended going to the 21 pitch prop. Im going to go down the my local (125 miles away) VP dealer this weekend and hopefully they will have one in stock and I'll put the original size back on it..

 

[Maclin]

It's a 2000 model which is carbureted, it also says on the data plate 4400-4800 now that I take a closer look.. Thanks for the info, The guy I purchased it from said it had a 14.3x23 size from the factory and he always carried a full load so someone recommended going to the 21 pitch prop. Im going to go down the my local (125 miles away) VP dealer this weekend and hopefully they will have one in stock and I'll put the original size back on it..

Cool, all that confirms you are on the right track to meet your goals. Should end up with a fleet little 17 footer

Happy Cruising.

 

[Leardriver]

That prop is perfect. Add a full load of people, a hot day, towing a skier, and you will prefer that prop over a taller pitch. You will never run wide open anyway except to test a new prop.

 

[Sea Rider]

Test prop that was used with a heavy load but lightly loaded, post achieved, max wot revs, won't blow the engine, it's just for a short time run. If the original prop, surely a medium pitched one is available, check wot rpm too, stay with the prop that dials engine to run at least middle to max wot rpm range..

Happy Boating

 

[jimmbo]

Mercury makes some, actually most of the best props for recreational boats. I myself am using a Mercury Enertia on my VP and am very happy with it
https://www.mercurymarine.com/en/ca/propellers/all/
https://www.mercurymarine.com/en/ca/propellers/comp-series/enertia/