Prop choice when repowering - from single to twin engines

[SeLaOffshore]

Prop selection is driving me nuts! I’m just about done converting my 1995 Proline 240s (24 ft hull - 3,600 lbs) from a single 2003 Merc - 225hp Optimax to twin 2003 Merc – 225hp Optimax. The single engine was propped with a 4 blade – 14.75(diameter) 17 pitch prop. Maxed out ~5,200 RPMs and 47 Mph. Cruise speed was 32-34 mph @ 4,000 rpms consuming a tad over 10gph.

Everything I have read says I should increase the pitch on the props to take full advantage of the twins – for speed and efficiency. So I consulted with Powertech Props and they recommended a pair of props that will just about require a 2nd mortgage on the house – priced at $710 ea!

I do believe stainless is the only way to go, but I’m trying to be real about this and would really appreciate some reasonable advice on how to distinguish my optimum choice where fuel efficiency for long runs and returning home on 1 engine are my highest priorities – 3 or 4 blade, 19 or 21in pitch, cup or no cup?

 

[jestor68]

Re: Prop choice when repowering - from single to twin engines

Mercury says that when going to twins; to add 4 inches of pitch.

You might consider trying aluminum props until you get the size just right. Solas aluminum props are only $118 apiece and the performance might surprise you. Just about any props you buy these days are cupped; some are cupped more than others.

Stainless props are too expensive to experiment with, unless you have a shop/dealer that will let you try different props with the understanding that you'll by from them.

 

[JoLin]

Re: Prop choice when repowering - from single to twin engines

Sorry for going off topic, but I'm concerned- what have you done to reinforce the transom? The boat is 18 years old, you're doubling the engine weight and overpowering by 50% above Pro-Line's specs (max 300 hp).

 

[JoshOnt]

Re: Prop choice when repowering - from single to twin engines

I would say out of the options you gave to go with a 4 blade, 21p and cupped **most are already cupped though now days.

Personal opinion would be 4 blades, 23p and cupped, that is because I like to run at 85-87% RPM

 

[SeLaOffshore]

Re: Prop choice when repowering - from single to twin engines

No problem with the question / comment JoLin - it is a very serious point that I have considered, and I appreciate that a reasonable consultant would ask.

Yes, I am overpowering the boat vs. the official hp rating, but not without a lot of research and consultation with experts. My objective in twin motors is redundancy for a safe return home - not top speed, and to keep my costs to a minimum. I found a great deal on a motor that exactly matched my existing motor while looking at a variety of alternatives. This choice was the most cost effective.

I did pursue installing additional knee braces for reinforcement with a fiberglass boat specialist (25 yrs experience) - He said with the quality of that transom -I'd be wasting my money and his time. He said that transom is built plenty strong enough to handle the project. On that model, Pro-line used a very thick composite structure (no wood) in the transom. The boat builder shop that custom built the transom bracket also commented about the very high quality and strength of the transom.

Insofar as the weight - I've seen dozens of the same boat with twin 150 Mercury 2 cycle outboards which weigh 422 lbs ea. My 225 Optimax are 477 each. The total weight over market experience is 110 lbs. Both the boat builder and fiberglass man I consulted felt this would be an acceptable increase. I also had a conversation with a Pro-Line rep telling them of my plans and while he could not officially "sanction" the effort, there was no mention of concern for the strategy.

 

[SeLaOffshore]

Re: Prop choice when repowering - from single to twin engines

The Mercury guidance to add 4in of pitch is interesting. I have been leaning to the 4 blade 21 pitch and that would be exactly consistent with the Mercury guidance. I'm also intrigued with the recommendation for the 23in pitch - still assuming 4 blade. My single motor cruise speed has been ~32mph at 4k RPM. I'm guessing a 23p would provide for even lower RPMs and a higher cruising speed - and therefore keeping fuel consumption to a minimum. I realize that with twin 225's I'll have plenty of power, but should I have any concern about hole shot with the 23in pitch?

 

[SeLaOffshore]

Re: Prop choice when repowering - from single to twin engines

Thanks for your help fellow mariners! I've ordered a pair of Solas Rubex - 4 blade 14 in, 21 pitch propellers. The combined cost for these 2 props will be less than one prop recommended by PowerTech. After they've arrived and I've had a chance to pull some performance numbers on them, I will post here for the final feedback.

 

[JoshOnt]

Re: Prop choice when repowering - from single to twin engines

I would say with the 23p you would have no problems with hole shot unless you overload the boat with dead weight. So inotherwards don't put everyone right at the back with all gear ect.

 

[SeLaOffshore]

So the search for props are long behind me. The Solas Rubex - 4 blade 14 in, 21 pitch propellers I installed have been exactly what I needed. I have a great hole shot regardless of weight or weight distribution. Cruise speed is very dependent on sea conditions, but on calm seas I'll run between 42 and 43 mph at 4k rpm and fuel consumption is about 10 gph per motor, 20 gph total. So theoretically at 20 gph and 42 mph - I'm seeing better than 2 miles per gallon in a 24ft boat. Getting the boat to plane on one engine - in tests - is tricky but possible. Max mph is unknown and will never be attempted, because I find that the boat handling starts to get real squirrelly around 55 mph - I think because the boat is simply not built with the type of aerodynamics needed for that type of speed. (BTW there was still plenty of throttle/rpms left at 55 mph). Thanks for all of your input and feedback on the prop selection process - a couple of years ago. Now it's on to the challenge of keeping these 14 yr old motors in optimum running condition.

 

[jimmbo]

Re: Prop choice when repowering - from single to twin engines

The Mercury guidance to add 4in of pitch is interesting. I have been leaning to the 4 blade 21 pitch and that would be exactly consistent with the Mercury guidance. I'm also intrigued with the recommendation for the 23in pitch - still assuming 4 blade. My single motor cruise speed has been ~32mph at 4k RPM. I'm guessing a 23p would provide for even lower RPMs and a higher cruising speed - and therefore keeping fuel consumption to a minimum. I realize that with twin 225's I'll have plenty of power, but should I have any concern about hole shot with the 23in pitch?

You still have to prop them so they run in the WOT RPM range. And yes with two engines you will run higher pitches than with a single engine. Then If you need to run on one engine because of failure you will have too much pitch. Is the second engine a counter rotator?

 

[SeLaOffshore]

Yes, the second is counter-rotating. Port - clockwise, Starboard - counter clockwise.

 

[jimmbo]

Yes, the second is counter-rotating. Port - clockwise, Starboard - counter clockwise.

I think you've got them mounted backward. The counter rotating one should be on the port side and the regular rotator on the starboard side

 

[ondarvr]

It can be done either way depending on the desired result.