Prop recommendations

[speedmerchant]

Hi fella's,

Brand new to this forum, and think it is by far the most informative boat site on the internet. Great job.
Ok so here's the scoop. Bought a 21' Chris Craft last fall and had it out for the first time on the water last weekend. While it hauls the groceries ok I would like some more top end. The details are below

Boat: 2000 21.5' Chris Craft 215 sport cuddy
Weight: 3540lbs (empty) guessing around 4400lb for an average load
Engine: Volvo Penta 5.7GS (250hp)
Sterndrive: Volvo Penta SX-M with a 1.51 ratio
Current Prop: 14 1/2 x 19 aluminum
Hull: very good to new condition

The boat currently does about 48-49mph @ 4650-4700rpm with the banged up aluminum prop. Recommended operating rpm is 4400-4800 for this motor. I will definately buy a stainless prop but am wondering what prop pitch/dia might get me to the 55+mph area. Volvo lists a 14 x 21 which is what I am thinking might do the trick. 1/2" smaller diameter will increase the rpm but the more agressive pitch should bring it back down to keep it within the operating rpm range? Any help is greatly appreciated!

Cheers,

James

 

[Dhadley]

Re: Prop recommendations

If I was propping it out I'd sure give the new Turbo Vector a shot. It's 14 1/2 x 21 and should do the job nicely.

 

[hwsiii]

Re: Prop recommendations

Speed, is that 49 MPH a GPS speed or a speedometer speed.

H

 

[MikDee]

Re: Prop recommendations

IMO, Thats a lot of boat for that engine, I'm surprised you get 48-49mph out of it? I doubt you'll ever get 55mph out of it, unless you mod that engine. Anyway, a 21" pitch aluminum prop will drop you about 400rpm, probably taking you out of the recommended rpm range, and loading up the engine. I recommend a good 19" pitch stainless prop instead, that should at least take you over 50mph, pretty respectable.

 

[speedmerchant]

Re: Prop recommendations

Speed, is that 49 MPH a GPS speed or a speedometer speed.

H

Was 49mph on the speedometer. I spoke to another fella with the same boat/engine configuration and he told me he gets 47mph on GPS. Thanks in advance!

 

[hwsiii]

Re: Prop recommendations

In my opinion a 21" pitch prop will lower your RPM to about 4,200, but if it has the right geometry it could be more than that, but with the wrong geometry it could be less RPM in stainless.

H

 

[speedmerchant]

Re: Prop recommendations

so what is your recommendation for more top end?

 

[hwsiii]

Re: Prop recommendations

Speed, the only way I could really recommend what I consider the right prop for your boat would be if you had a true GPS speed at WOT settings and run two tests back to back in opposite directions add the two numbers together and divide by two. That is the only way to know the true prop slip you have right now, and that is a key to figuring out the blade surface area and geometry needed for heavier cruisers with a lot of prop slip.

H

 

[speedmerchant]

Re: Prop recommendations

Ok thanks, I'll do exactly that this weekend and let you know.

 

[MikDee]

Re: Prop recommendations

If your goal is 55mph you'd need to put a performance cam in that engine

 

[Gr8Lakes]

Re: Prop recommendations

Short of a new engine, I have heard that a DPX Duoprop lower unit will give big performance gains, and will bolt right up to what you have. A used dpx lower might run 2k or so.

 

[speedmerchant]

Re: Prop recommendations

Ok, sorry for the delay guys. Just came off the lake this afternoon and have the final GPS numbers and RPM. Tested during multiple runs, and was identical every time spanning a 30 minute trial.

The boat does 49.9mph at 4875rpm. I friggin tried for that extra .1mph let me tell you!!! This is GPS verified. So same as before, looking for a bit more top end in the 55mph+ range. Considering the current aluminum prop and it's not so great condition, 55+ shouldn't be out of the question with a stainless and pitched and/or diametered right. So, again, am I close in thinking I could go with a 14 x 21 stainless? Going by the rpm drop rule of thumb going with a 21 would decrease rpm to the 4500rpm range. So will the slightly smaller diameter (14 vs. current 14 1/2) bring the rpm back up to the high range near or about 4600-4700rpm. Look forward to hearing your further comments! Don't forget the above mentioned boat/motor details. Thanks.

 

[speedmerchant]

Re: Prop recommendations

bump. anyone?.....

 

[MikDee]

Re: Prop recommendations

Hey, you're doing quite well as it is IMO, Yes, A 21" pitch SS should raise your top end a few mph, and drop about 400rpm, but the 1/2" smaller diameter doesn't really matter that much. An inch smaller maybe, but then your prop might be too small to give you effective thrust.

 

[speedmerchant]

Re: Prop recommendations

Thanks Mike for the input. Had a local dealer here recommend a Mercury Marine Mirage Plus prop in a 15.25 x 19 and told me I would never turn a 21" prop. I won't say what I think of most dealers, car or boat but is this other size a viable option? Does this seem right? Seems the diameter is getting rather big and would have to check it will clear for fittment. Of course the price of the Mercury props are extremely pricey. Is this another option?

I am getting real eager to pick a prop, and don't want to keep running the current imbalanced funky one.

 

[MikDee]

Re: Prop recommendations

Mirage props are good, but one that big in diameter is not needed on your 21' boat. I would think a Volvo 14"X21" is a good starting point, but IMO the choices here at iboats are better, I can't find your drive in the index? but have a look above the prop forum for choices, I recently heard of a member here having good luck with a Solas 3 blade SS. Or a Hustler aluminum is a good choice.

 

[Tail_Gunner]

Re: Prop recommendations

If I was propping it out I'd sure give the new Turbo Vector a shot. It's 14 1/2 x 21 and should do the job nicely.

............

 

[hwsiii]

Re: Prop recommendations

James, sorry I haven't gotten back to you sooner. I have been studying your numbers with the current prop, and I believe it is skewing the numbers that you ought to be getting with that prop. I know this won't make you happy, but my recommendation is to bring your current prop to a REPUTABLE prop man and have him fix the prop to factory specs or buy a new 19" aluminum prop exactly like you have and let's rerun the prop and see what the new numbers are, you need a SPARE prop anyway and that will make certain that we have GOOD numbers. I do not believe the numbers we have now are really reflective of an aluminum 19" prop on your boat. I know how it is wanting to get the most out of your boat and motor and how it is hard to wait to get there, but that is my suggestion.

H

 

[speedmerchant]

Re: Prop recommendations

Thanks MikDee, hwsii and all,

I'll try and see what I can come up with. The current aluminum prop isn't as bad as you may be thinking hwsii. It definately has a slight "warping" look to the tips along with some nicks where the previous owner had contact with something. I don't know how much of a performance loss there is with it. Thanks to all for their recommendations and thanks H for trying to run some numbers.
I think I'll just order a bunch of props and start testing. Probably the best way to get an idea.

 

[hwsiii]

Re: Prop recommendations

James, if that is your decision then I would recommend buying from either Dan's Discount or Propgods as they both have an exchange program for testing props and have a great range of props they carry. I have also heard that Iboats has an exchange program available as well, I just don't know the actual guidelines for it. Just make sure how many exchanges you are allowed and exactly what the cost is for each prop, including shipping. My recommendation would be a Michigan Apollo 14" x 21" pitch. It is a high rake, progressive pitch prop with cupping and should help with the prop slip you are experiencing right now as the cupping helps control the water coming off the prop. And you can purchase it for less than $ 250.
If that doesn't do the job then I would take Dhadley's advice on the Turbo Vector 14 1/2 x 21" pitch.
James, I wasn't worried about a performance loss with the old aluminum prop, I was worried about a gain in RPM that is not commensurate for the pitch of the prop itself and thus the increase in prop slip. When props have had something happen to the blades by hitting something you never know which way the prop will react, either it can increase the RPM or it can decrease them, I have seen it both ways.

H