prop/motor location

[forceproblem]

Hello I bought a 1994 18' maxum bowrider and it came with a 2003 optimax 135 I used it all last summer and love it over my old force 125. the question I have I was looking at putting a stainless prop on and as I look at other threads I hear of people moving there motors up a bolt hole or some up to three. Correct me if im wrong but isnt the cavatation plate supposed to be as close to flush with the bottom of the hule as possible?? Ok then for the prop I have a 17 pitch aluminum prop I am fairly happy I would like a little better whole shot but am happy withit other wise I run at wot 5450 to 5500 of course that is as exact as my gauge tells me I would like to be recomended a good quallity stainless prop thank you

 

[Dhadley]

Re: prop/motor location

How high your motor will end up (X dimension) is determined largely by the technology built into a prop and how it reatcs with a given hull deisgn. SS props will generally run higher than aluminum props and 4 blades generally run higher than 3 blade props.

The theory behind this whole thing is to reduce drag. The higher we can run the gearcase the less drag we have. That translates to improved efficiency.

 

[Maxz695]

Re: prop/motor location

A 19 pitch of the same diamiter would cut back your RPMs to a range of possible 4800 to 5000 rpm. You would get more top end speed at the cost of a good holeshot and getting up on plane taking much longer, also it would be stressing the motor more than it should be. If you bring others and alot of gear on the boat. I wouldn,t recommend the added stress to the engine. lowering it to a 14 P or the like will cause the engine to over rev and do damage to it. If your engine top RPM range is 5500 I would keep it right where it is. JMO You have the option of using a 17P stainless prop which will help some or you can port the aluminm prop to give you a better holeshot View attachment 180153

 

[jestor68]

Re: prop/motor location

Putting the same pitch SS prop on there will lower your WOT rpm by about 200 rpm if you leave the motor at the present height.

Raising the motor one hole will gain back the rpm(less drag) and result in an overall performance gain across the board.

 

[dan02gt]

Re: prop/motor location

How heavy is that boat? I would think a 135 Optimax would spin a 17P aluminum prop to the moon on a 18' bow rider unless it's really heavy. Is the prop in good shape? What speeds are you getting (GPS)?

Also ignore all the mess about the anti cavitation plate hight. On a boat that size you should measure prop to pad relationship for the best setup How to measure prop to pad height

With that boat and no jack plate you would want the prop shaft about 5" below the pad to start and adjust from there.

 

[forceproblem]

Re: prop/motor location

Ok I understand location better now when I bought the boat it had a better hole shot and top speed but when I was wanting to corner sharply the engine would rev way up so then I lowered the motor I went down to where the cavatation plate was level with the hull lost performance that way but I could obviously bite into my corners much better. Givin the info I have received I am in need of adjusting my height I believe I went down 3 holes and maybe should just start by bringing the motor up 2 holes and see how that is. And my prop is in excellent shape I just would like stainless ss prop for its performance reasons I only boat in lakes and am not worried about hitting anything

 

[dan02gt]

Re: prop/motor location

Look into a Tempest or Trophy Plus for your boat. Either one will be alot better than the prop you have performance wise and will not blow out near as bad in corners. The Tempest is designed for heavy boats and may offer the best performance of the two.

By lowing the motor you buried the prop and probably killed your performance. So raise up a little and measure using that link I posted.

 

[jestor68]

Re: prop/motor location

Your boat has no "pad". Standard mounting height is most likely hole number two. You want to start with the AV plate 1-2 inches above the keel(bottom).

Your problems stems from the fact that the standard Merc aluminum prop has little if any cup; that's why it ventilates in turns. When you turn, the boats leans into the turn, bringing the prop closer to the surface, which invites ventilation.

A SS prop with a decent amount of cup in the blades has better grip, resisting the tendency to ventilate.

A Solas HR Titan 14-3/4 X 17 will perform well on your boat.

 

[forceproblem]

Re: prop/motor location

The av is the cavatation plate is this correct

 

[dan02gt]

Re: prop/motor location

The av is the cavatation plate is this correct

Yes one in the same. You can also use the lowest point on the keel to measure if your boat doesn't have a pad.

As far as props go stay away from any prop that doesn't have a exhaust venting system. It doesn't appear that any of the Solas props have this but I could be wrong. Mercury props use the PVS (performance venting system) to allow you to adjust how much exhaust is directed over the blades of the prop. This allows your motor to spool up and get you out of the hole sooner. Your Opti will like to be tuned to around 3500-4000 RPMs on the hole shot. Read about it here: PVS - Performance Vent System | Mercury Marine Trust me it make a big difference with a 2-stroke outboard.

A good prop shop can add them to any SS prop BTW.