Armstrong Bracket/Engine mounting question??

[fowl intent]

The boat is a 26 foot SeaSport, 3400# deep v, with a factory mounted Armstrong bracket. The Merc 225 EFI is mounted to the bracket with only an additional 3/4" or so for downward adjustment. As it is presently mounted the cavitation plate is a full 6" above the lowest point of the v hull. Is the very lowest point of the hull the reference point? If so, then it would seem that this is a design flaw, unless the bracket was designed for a long shaft (if there is such a thing in a 225 hp). The only way I see to lower the motor more than 3/4" is to have someone cut the top 2-3" of the Armstrong bracket, but that still wouldnt get the cavitation plate flush with the bottom of the hull. Does the fact that the motor is actually mounted on a bracket some 2+ feet behind the transom change the standard rule that the cavitation plate should be even with the bottom of the hull???

 

[JB]

Re: Armstrong Bracket/Engine mounting question??

Howdy FI.<br /><br />How far back from the transom does the bracket place the engine?<br /><br />As I understand it, the engine should be raised an inch for each X inches of setback, so that the AV plate meets the water rising from the transom.<br /><br />There are several others here who can advise you of the exact "X" imension you need for a given amount of setback.<br /><br />DHadley should be along soon.<br /><br />Good luck.

 

[Kenny Bush]

Re: Armstrong Bracket/Engine mounting question??

Sounds like it is pretty close. I have heard most people say, to raise the motor an additional 1" for every 12" of setback. So 24" bracket would mean an additional 2 inches of lift. So that says your cavitation plate should be 4" above the keel. That is about where the cav plate is on my Fountain. The real test would be to get the boat on a good plane, and see where the cav plate is running in the water. If your cav plate is in or under the running surface, it will need to be raised. If the cav plate is above the water, you will probably need to lower it. If the cav plate is running right on top of the waters surface, thats perfect............

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

Would a change in prop, without lowering the motor, reduce tendency to cavitate? I have a 14 1/4" x 19 Rampage SS, and have been advised that a 4 blade would probably help that situation. I hate to sound so ignorant about these things, but....I am.

 

[Reel Appeal]

Re: Armstrong Bracket/Engine mounting question??

It may help,yes.The prop you are using now is of too high a pitch for your 3400lb. boat.This could also explain the spun hub.I have a 22'Aquasport(approx.2600lbs.)w/a 15x17 prop(225hp).Are you reaching around 5500 rpm(full throttle)?If not try a lower pitch prop.Your local marina may let you borrow a test prop if your intent is to buy a new one with them.Personally,I do not think a four blade prop will solve your cavitation problem.Good luck.

 

[lakensea]

Re: Armstrong Bracket/Engine mounting question??

Lowering the engine 3/4" will make a big difference - it should be all you need. It also should lower your WOT RPM by about 100.

 

[Kenny Bush]

Re: Armstrong Bracket/Engine mounting question??

Man....Don't you change anything until you see how that cavitation plate is running on the surface..... There are a lot of factors to this formula. You need to get the motors set correctly(if they are not already). Work the props later.

 

[wilde1j]

Re: Armstrong Bracket/Engine mounting question??

Is the motor cavitating? If not, don't change a thing.

 

[Dhadley]

Re: Armstrong Bracket/Engine mounting question??

Is cavitation the issue or do you need more rpm? What is your top rpm with an average load? What spun hub? Are there two threads on this?<br /><br />Lets start with the basics - hull designs and transom angles vary but as a rule of thumb we find you can go up 1" for every 5-6" you go back. You say you are back 2+ feet. Again, generally speaking, standard brackets are somewhere about 28-30" back to allow for full tilt. That puts us in the 5" range so you are close. Now we look at prop design. Yes, a good design 4 blade should grab more water than a 3 blade of the same general design. IE - 4 blades tend to run a higher X dimension than a 3 blade of equal technology.<br /><br />If the hub did spin and you have a 3 liter Merc, thats not that uncommon. Did it just spin or did it look like it burned the rubber hub?<br /><br />At this point I would say the guys are right - dont change anything until you give us a little more to go on.<br /><br />Good luck!

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

Dhadley, I was hoping you would chime in here, and thanks to everyone else for their opinions. The original problem was that I noticed fairly frequent occasions where the prop was cavitating, particularly when cruising with a following sea. Then several weeks ago I spun the prop while cruising down the waterway (didn't hit anything that I am aware of). I had a replacement hub pressed in, and the first time out I experienced the same cavitation problems. I am concerned that I will have another failure, this time 18-25 miles offshore rather than in the waterway. Thats the history. As far as RPMs at WOT, I will try to get some idea of that in the next few days, and the same time, I will try to determine where the cavitation plate is in relation to the water at cruising speed. Thanks again for your time.

 

[Dhadley]

Re: Armstrong Bracket/Engine mounting question??

What brand / type of prop? Do you remember if the hub looked burned?

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

The prop is a Michigan Wheel "Rampage" Stainless Steel, 14 1/4" X 19. The hub had to be removed with a hydraulic press, and I didn't get to see it, not that I would have known what to look for.

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

OK guys, here's the data. With the above setup, I'm getting 5200 RPM at WOT, and she's doing 42 MPH (33 knots/hr.) At cruising speed the cavitation plate is just skimming the water, so that is apparently not the problem. Does this info help at all?

 

[Dhadley]

Re: Armstrong Bracket/Engine mounting question??

Thats about 22% slip which is quite a lot. It may be time to go prop shopping. But thats what you suspected all along.<br /><br />Just checking something here - the motor the is an EFI and not the OptiMax, right? Is 5200 the range you want for top rpm?<br /><br />Good luck!

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

Dhadley, or anyone else: what prop would size/pitch prop would you recommend to increase RPMs? Is there a web site that explains the relationship of prop size/pitch to performance (ie. increased top end speed/out of the hole performance/rpm??

 

[Dhadley]

Re: Armstrong Bracket/Engine mounting question??

Are you saying you do want more rpm?<br /><br />This is not an OptiMax - right?

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

Dhadley, I know the specs on the motor call for 5500 rpm. Do I want more rpms? If that will increase performance, top end & cruising speed, and solve my cavitation problems, then YES. I really am not knowledgable enough to know the relationship between prop size/pitch and performance. ie. would the slippage problem you noted be reduced by going with a prop w/ less pitch? more pitch? greater diameter?

 

[Dhadley]

Re: Armstrong Bracket/Engine mounting question??

We really need the basics at this point. Is this an OptiMax?

 

[Kenny Bush]

Re: Armstrong Bracket/Engine mounting question??

I would switch to the 2-piece solid plastic hub. That way if you spin, you can change the hub yourself. I have even changed one in the water. Anyway, the next time you spin a hub, when the prop shop presses it out, then switch to the 2-piece and buy you a couple of extra's and you won't need the prop shop pressout anymore.... As for your cavitation problem, sounds like it is going the be tricky... On my twins, I have a couple of sets of props that I play with. I have a pair of 23 pitch Chopper style props(Rear half of the blade is removed) which give a better bite and a little bit of lift in the rear. These choppers do cavitate in turns more than usual. The other set is a pair of 21 pitch mirage props with full ears on the prop blade and they are the ones I use most. They tend to hold the water best in all-around conditions. I have switched to the 2-piece hubs in all of my props and I love them. Those rubber hubs seem to blow on me every season, but I have been told my boat is heavy, and that porpoising is usually the culpret. From your post, I do NOT think you want a lot more RPM's. I think your are in the Zone, but maybe you could go another 200-400 RPMs. You may want to go to a smaller diameter, same pitch, prop. Go to a prop shop and get some loaners.. You may have to put up a deposit but it is cheaper than buying then trying. I like the choppers for offshore, and they cruise very well. In the slow swells offshore, the choppers give a little, as that bow rides up and down through the waves. A full eared prop, which bites/holds the water better, will bog the motor a little in the swells due to more traction.

 

[fowl intent]

Re: Armstrong Bracket/Engine mounting question??

The motor is a 2001 Mercury EFI Saltwater 225 hp.