Very confused prop

[charlessavage]

Man I cannot begin to tell you how confused I am but here is the information to start. I am the original owner of a 1973 Crestliner 23 w/hardtop. This is the 3rd engine drive change out all done by me. Spec is as follows 2003, 305/220 hp 2bbl carb. -brand new long block- used 1.5 ratio outdrive (Alpha One) excellent condition- 15 1/4 x 15 new prop - hull perfect - trailer boat- weight w/fuel and average load 5800lbs - 96" beam - 91" transom width -16" draft - deep v hull. The RPM will only go 3,950. I have moved to a new prop (15 ? x 13) and still get 3,950. I have checked and the carb is wide open. The RPM has been verified with a laser tach. The shaft alignment is perfect and timing has been verified as correct. The carb is new and the unit runs GREAT, with the exception of RPM. I could get 35-36 mph before and can only get 28 to 29 now. Any suggestions I am at my wits end.
Thanks CJ

 

[gcboat]

Re: Very confused prop

Your prop diameter really has nothing to do with increasing or decreasing the rpm's of the motor. Your prop pitch is the culprit. I believe the rule of thumb is for every 1 degree of pitch change you will see 200 rpm's of engine change. So you are going the wrong way in your prop choice to obtain additional rpm's.
So was that 15-1/4 wheel giving you the proper rpm's before?

 

[charlessavage]

Re: Very confused prop

I was running a 15 1/4 Diameter 15 Pitch and moved to a 15 1/2 Diameter 13 Pitch. The only reason I got a 15 1/2 was that was all they had in 2" smaller in pitch. I was afraid that if I moved down 1" in Diameter and 2" in pitch it would be WAY too much. The same prop was giving proper rpm with the other engine however it was the same hp but a 4 bbl carb and not a Vortex setup like current unit.
Thanks

 

[tmh]

Re: Very confused prop

Seems like a lot of boat for a 220 hp 2bbl carbed engine! I'm not too surprised at the performance #'s. Is it an aluminum prop also?

What were the previous engines? Why did you put in a 305 instead of a 350 like that boat needs (I think)?

I don't think you're going to solve this with props, unfortunately.

 

[charlessavage]

Re: Very confused prop

The previous engines were 188hp 302 2bbl / 220hp 305 4bbl and now a 220hp 2bbl 5.0/305. The prop has always been aluminum, I have never tried ss. It is a 3 blade prop,cupped.

Thanks

 

[steelespike]

Re: Very confused prop

Prove it is running on all cylinders,how do the plugs look?Check compression,any chance of a rubber exhaust connector collapsing?Is idle rpm drop ingear seem normal?Is fuel pump strong?Try carefully squirting gas into carb at speed.You might pull the outdrive and check for binding.Any chance a motor mount is loose or weak?

 

[charlessavage]

Re: Very confused prop

New plugs, old ones looked great only 1mo old but I changed to the EXACT plug recommended by Merc. Bellows new-Idle Perfect-Pump psi at spec-Motor mounts new and checked -Motor alignment checked and done w/merc. tool and slides in effortlessly - rear engine coupler check and is new- outdrive turns easily by hand. Ran the unit last Saturday for 6 hrs various speeds and load, wide open for about 4 hrs of that ran great.

Thanks for the reply
CJ

Almost forgot, engine is new but i checked compression anyway, all spec.

 

[MaxOD]

Re: Very confused prop

Might be a stupid question but you mentioned that the out drive was changed. Same ratio as the old one?

 

[charlessavage]

Re: Very confused prop

1.5 ratio on old unit 1.47 on new. Close enough I think to be called equal.
Thanks for all suggestions I am looking for any and all ideas. I talked to a prop guy today and he said that 1" of diameter is equal to 2 pitch sizes. So using that I am having a prop resized to a 14.25 diameter x 14 pitch. He was scratching his head as much as I am. We want to keep up the diameter as much as possible for a boat of this weight so this is the number we were going to try. I will have the prop by the end of the week and run it out.

Thanks to everybody and please keep sending suggestions
CJ

 

[charlessavage]

Re: Very confused prop

Well here it is - I proped the boat with a 14 x 14 prop and got 4175 at 30 mph. The boat seems to run much better but I still am not up to RPM min. I was thinking based on the current results if I should reduce the diameter or pitch or both? I am still looking for ideas.

 

[Boompa]

Re: Very confused prop

If you plan on pulling loads (skiers/tubes/lots of people) I would drop the pitch in favor of the larger wheel. If you were going for speed the opposite would probably be the case.

 

[charlessavage]

Re: Very confused prop

Thanks,
Just got another prop cut to 13.75 Dia x 13 pitch. Will run and post the results. Maybe I will find the sweet spot.

 

[steelespike]

Re: Very confused prop

I can see you want to use your boat so need to reach proper rpm but something "ain't" right.if you were getting 35-36.and reaching rpm ???
I know its a new rebuilt; perhaps valve timing ,perhaps valve lift? wrong or no timing advance.Coil polarity reversed?Too much oil, dip stick wrong or damaged,Crankcase ventilation plugged /pvc valve reversed.Flash supresser
or some hose etc blocking throttle linkage but not blocking when checking throttle opening.

 

[charlessavage]

Re: Very confused prop

The 35-36 was with a different engine and outdrive. This engine is brand new, not a rebuilt. RPM has been checked against the onboard tach with a laser tach and verified ok. Spark advance and all electronics were checked by computer with unit running at speed and checked ok. Dip stick is original but I put the oil in on startup and it was the right amount according to spec. not the stick. PVC valve is fine and there is nothing anywhere around the throttle and it is open to the stops.
Thanks for response.

 

[charlessavage]

Re: Very confused prop

Well here it is, the 13.75 x 13 did 4500rpm @31mph. Great on the rpm but need a little more diameter for the seas and load - next step 14 x 13. I think I have found the sweet spot!

 

[branman1971]

Re: Very confused prop

I would think you would go to a smaller diameter prop now instead of going larger? Your RPMs are only at 4500, and I am told by my local boat shop that WOT should produce 5000 to 5500 rpm. I expected you to say that you were going to a 13.00 x 13 maybe. I am very interested because I am about to go through this prop hell with my pontoon boat.

 

[charlessavage]

Re: Very confused prop

My 305 Carb spec is WOT of 4400 - 4800 RPM. The diameter is very important on a heavy deep v hull and I have learned a lot about rake, pitch, thrust and diameter of props and the effect. The problem that I think I would have with 13 x 13 is the thrust of the prop would not be sufficient to raise the boat on plane in a reasonable time. My engine is an inboard/outboard and I assume that you are running an outboard. Outboards run higher RPM?s and have a different torque curve than a V8 inboard. If you would like to talk let me know and I do have some extra props (if they would fit your application) to help you out.
Thanks

 

[charlessavage]

Re: Very confused prop

Just for anybodys information:

Blade Rake represents the angle of attachment of the blade to the hub of the propeller. This is not to be confused with the pitch, which is a measure of the twist or screw progression. Higher rake normally improves performance in ventilating or vavitating situations (high engine elevations and high trim angles). Additionally, higher rake can provide higher bow lift, which will frequently improve speed. Low rake blades are typically used on motors with propellers running fully submerged, typically carrying moderate to heavy loads. The rake angle can either be straight or the average angle of a parabolic curve

Constant pitch propellers have a constant pitch across the entire propeller high pressure face. Progressive pitch propellers have a lower pitch at the leading edge which increases progressively across the blade face. This type of propeller typically provides superior planing performance.

Cup assists in increasing the water gripping capability of the propeller, particularly on boats with elevated motor heights and high trim angles. Cup can also provide additional bow lift when utilized on the rake line of the prop. Applying cup to the trailing edge of the prop along the pitch line will increase the effective pitch of the propeller. A standard cup will typically result in a decrease of 200 to 400 rpm's. This usually means a decrease in pitch of 1 to 2 inches is required to run a cupped propeller in place of an uncupped wheel.

 

[charlessavage]

Re: Very confused prop

I GOT IT!!! Here are the results:
35mph (GPS)
Stainless Steel
14" diameter
13 Pitch
Vented blades
High rake
4450 RPM

THANKS FOR ALL THE HELP

 

[Irv964]

Re: Very confused prop

I GOT IT!!! Here are the results:
35mph (GPS)
Stainless Steel
14" diameter
13 Pitch
Vented blades
High rake
4450 RPM

THANKS FOR ALL THE HELP

What SS prop do you choose? Congrats on getting it all worked out.......Irv964