Prop Question

[blubeast]

I'm considering changing the prop on my 21' cuddy cabin. I'm currently running a 15 1/2 x 15 aluminum prop on the OMC i/o powered by a 4.3l v6. It has power trim tabs.

With 2 adults on board, gets on plane fine... 4 or more it is slow and very hard to reach plane, even with the tabs. I don't care about losing top end speed... just want to improve the hole shot to get on plane faster.

Has anyone changed their props out? Would changing to a 15x15 stainless be enough of a change? What about switching to say a 14x19 comprop?

Thanks in advance for your suggestions.

[strgnslyr]

i run a five blade merc hifive on a 20' intruder
jumps right up on a plane

[Miss B Haven]

blubeast - don't mess with the diameter (much).You can get more slippage (cavitation) if you reduce the diameter. Also - that 14x19 will be more prop than you currently have and make the situation worse. Reduce the pitch, don't reduce the diameter. Dropping to 15 from 15 1/2 isn't much of a change. You can probably get away with that just fine, but I wouldn't go down to 14.
The thing to do is to see what your achievable top end rpms are. You need to be able to achieve the rating of the motor within a couple hundred rpm. If not, drop the pitch. 1 inch in pitch should give you a couple hundered more r's. If you are able to make your top end then there's not much you can do about the fact that it's slow coming on plane when loaded. I suspect however you are not making top end.
A SS prop will help some (less blade deflection -but this is more important for top end speed) but I doubt you'll even be able to notice. Be aware that other than the initial cost it is more likely that if you hit something big you may break the stern drive rather than just take the prop off. Also - Alum props can be repaired for around $75 bucks. A SS will be 3 times that if repairable at all. If you dump $300-400 on a SS prop and then find it didn't fix your problem your out a lot of trial money too.
It sounds like you really just don't have the HP for the size of boat and loading (I've had the same problem in the past). Propping down (on the pitch) is the best you can do short of putting in more ponies and/or going with a duoprop (more efficient). Try a 15x14 or even 15x13 (or 15 1/2 dia) if your top end r's are WAY down.
PS - Last I saw - comp props are only good to a certain hp. I would think if your really pouring the ponies to your boat you would get more deflection on them and chance shearing a blade. They are really made (IMHO) for folks who are running in areas wit ha lot of "stuff". They shear easily and don't hurt the rest of the drivetrain. They are also cheap to replace (but cant' be repaired).
Good luck- been there, done that, learned to live with it till I could get a new boat!

[ 01-09-2003, 08:47 AM: Message edited by: Miss B Haven ]

[SureSet]

The amount of prop change that you need is dependent on a number of factors. If you look around on the web for prop shops, you find some have a calulator to help you determine what to get. Going to a 15x15 souinds like too small a change, and a 14x19 sounds like too much pitch to me, but I have no experience with larger boats. Talk to a prop expert.

SureSet

[Red]

I agree with the above comments. You need to go down on pitch, to give you more low end torque. IMHO Your boat length, weight, hull configuration sounds like it may be underpowered with the 4.3 L engine. There are tons of underpowered craft around, but that doesn't mean you can't impove your performance. My suggestion is to find a prop shop that will allow you to try several props, if the first one you choose, doesn't improve your performance. Performance is governed by physics, which can be explained mathamatically, but there is some trial and error involved here. Good luck. BTW, are you using the trim tabs correctly? Not trying to be a smart- aleck just would hate to have you mess with the prop if something else is the problem.

[Miss B Haven]

Blu- Moving people or gear forward to get on plane is NOT an unusual situation (not in a marginally powered boat anyway). Not sure your going to gain much if anything with a prop change if you're making redline. Getting on plane is a matter of getting the boat up to a certain speed. If the engine is reving properly then you may actually lose some ground by propping down.
If this was a car or truck, you can always just gear it down to get up that hill. The friction involved in moving the vehicle forward does not change appreciably with speed so if you want to haul a heavy load uphill with almost no power you can just gear down and slow down.
The dynamics involved with a planing hull are much different with 3 completely different friction aspects. Planing is a lot more like flying than rolling on the road. You have to reach that escape velocity. Imagine the worst case, propping down so far that the engine revs to redline almost immediately, but the boat never moves forward fast enough to get up on step.
If it really bothers you in the current config I would spend the $150 on that lower pitch prop and try it. If it doesn't help you at least have another spare prop. Personally I don't think it will help or hurt much for getting on step. It will cut your top end at a given rpm that's for sure. I have heard that Merc has a "try a prop" program but not sure about other brands or the details. Good luck (still haven't looked for those San Juan books - I'll try to remember tonight).

PS- What's the hull config (deep vee?) horspower and (full load) displacement of your boat?

[Red]

Blubeast,

I've had plenty of rides that require people to load the bow to get up on step, " it's all good ". Sounds like you've got a stable craft, that should provide plenty of fun. I don't have trim tabs, but I can advise that you should have them and your outdrive trimmed in or down , to get up on plane.

[Pilar]

Hi Bluebeast .... Cascade Marine on 149th and Sandy will let you 'Borrow' a prop to try. They have SS and aluminum. Problem is they are a Merc shop.

Does anyone know an OMC shop that will do this?

The other thing is in regards to SS vs. Aluminum. I have to agree with the thought that the soft metal on the aluminum will act as a 'fuse' and spare the very expensive parts in your outdrive if you hit something.

We are not bass fishing so the Nascar speeds that require SS prop do not apply here.

I have heard that a prop can be vented to give a better hole shot. I've seen a prop with a 3/8" hole drilled through the hub at the base of each blade.

Does anyone know about this?

[Miss B Haven]

Pilar- Nope and don't you be experimenting on Miss B's props either. :shocked:

[blubeast]

THANK YOU for all the suggestions/comments. That's why I love this site... very knowledgeable folks very willing to help.

Thanks again.

[blubeast]

Appreciate all the feedback... RPM with the current 15 1/2 x15 is right at redline at wot... so that appears to be the right config.

Regarding the tabs Red... fair question. Granted I'm still learing the boat, but honestly believe I'm using them correctly. At least I sure hope so...

Mel, got the comprop as an emergency spare... real cheap. Suspected it'd be marginal in primary use, but ok in an emergency.

Bought the boat used, so did not have any say in the engine size. Overall, it is very economical and performance is fine EXCEPT when getting in plane with a load of passeners.. guess I'll just not be able to take too many perople fishing... or at least go have them all sit on the bow till we're on plane.

Thanks again for the suggestions.

[ 01-09-2003, 12:25 PM: Message edited by: blubeast ]

[SureSet]

Redline at WOT is only the "Right" config if you are configurign for top speed and (at least with a 2 stroke) max engine efficiency. To configure for max holeshot, the "right" config would be one with less pitch. It will get you and your 4 people up faster, but obviously, then you cannot opperate at WOT as max RPM will be acheived prior to WOT. The key is that there is nothing wrong with either config, as long as you don't run push it over max RPM.

SureSet