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Discussion Starter #1
Hello All:

I have 28 foot Bayliner with a 7.4L Mercruiser and a Bravo II leg. Several weeks ago I was out fishing off Thrasher and we got our downrigger cable tangled in our prop. I was able to get it untangled and called ourselves lucky, called it a day and headed for home. The next weekend I went out with my buddy in some pretty strong winds and found we could not get any speed on the boat. Finally figured out that we had a spun prop. Long story short, we limped back to port and I bought a brand new stainless steel prop to replace the spun aluminum prop. Took the boat out after replacing the prop and now I seem to be limping along still. The prop does not slip like the spun aluminum one but I can't get the same speed that I used to. I used to be able to get 18 knots at 3800 rpm whereas now I can only make 11-12 knots at the same rpm. Stainless prop are supposed to be more efficient than aluminum, not less. Had the engine checked out make sure I wasn't losing power and everything seems OK. Any ideas? I did read that sometimes you need to replace the thrust washer when you go from aluminum to stainless but my mechanic said there is only one thrust washer available for the Bravo II on my boat. Any help would be greatly appreciated.
 

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I'd go get the hub on the old aluminum one replaced and keep it as a spare - its not that expensive.

SS props of exactly the same configuration (ie number of blades & blade type) and specs (diameter and pitch) will outperform aluminum always. They dont flex the way alumimum does (which is why the "effective" pitch the engine is seeing is often higher with SS than what you'd have seen with aluminum).

(pitch, in case anyone needs to know, is the theoretical distance a propeller would travel through a solid medium in one complete revolution - with no slippage; a 21" pitch prop would move 21" forward - a 19" pitch prop 19" which means, at top end, in theory, you'd go faster with the 21 than the 19 at the same RPM IF you had the horsepower to do it; what often happens is people add pitch looking for speed (espacially with smaller OB powered boats), and in doing so, dont let their engines get to their design speeds - which creates "lugging" and will wear their powertrain out prematurely)

My questions, what were you getting top end out of it with the original wheel? speed & rpm? is the new SS one the same type, pitch and diameter?

My hunch, based on what you're saying is that you're over-propped now, likely because you went larger diameter, or more pitch, or have stepped to a four blade high thrust prop - that your not able to generate enough horsepower to get your boat to climb onto a clean step and are stuck in a dirty (bow high) plane. But, im guessing really, I dont know your boat type well enough to know for sure. 3,800 seems way low to me - I'd have expected you'd at least be getting into the mid 4's if not low 5's at WOT (although, i'm used to OB's getting into the 6,000-6,400 range).

If you get it figured, please do post a follow up, curious about what the answer was/is.
 

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Hello Chewy:

Thanks for the reply. The new stainless steel prop is exactly the same size as the original aluminum prop (18.25 D x 19 P). I wondered if the mechanic changed the pitch myself but that is not the case. I did some investigating myself and found that on some boats a stainless steel prop may not go far enough into the leg hub and this will allow exhaust gas to leak onto the prop blade faces causing ventilation. I askd the mechanic about this so he checked it out today. This does not appear to be the issue.

HOWEVER........ in looking into this he did notice that the back of the leg hub is significantly scratched up where the downrigger wire got tangled. He now suspects that I might have damaged a seal on the leg housing that forces the exhaust gasses through the center of the hub and prevents gases from leaking onto the blades and that this is what might be causing the loss of thrust. I now have to contact my insurance agent as the fix will be to replace the leg housing (or the entire leg as that might be cheaper). Sure hope this is covered! Will probably need to haul the boat out next week so that a thorough inspection can be done to complete the diagnosis. Will let you know what I find out but I can gurantee you it won't be cheap!
 

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Klessig, read my post on hitting a log. Ended up with the same result as you. Going to mention to my mechanic what you said. I'll let you know what he says.
 

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Went back to the boat today and the mechanic gave me one prop to put on one engine (put it on starboard) and told me to take it out and go through the trials again. He said once out on the water with the engines running, attach a line from the positive side of the port engine coil to the positive side of the battery. This I did and running only the starboard engine (new prop) - no difference. Running only the port side (new wire) no difference. However, running both together made the port engine blow right past the 3800 rpm barrier but the starboard still was 3800 bound. This tells me that the port engine got a boost from the starboard engine which enabled it to run right up to 4400 rpm with throttle to spare. But how and from where?
The hit must have jarred something wonky. Also checked my grounds and will add new temporary ground lines from battery to engines and see if that helps



Nothing more fun than trying to sort out an electrical problem.
 

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Klessig,
just a thought, was your original prop ever reworked?
dont know if this still happens or not but about 10 years ago I had a prop recupped at Bridgeview marine
now had the next owner not known this and had to replace a prop with the same he would notice a significant loss in power as the new prop would have a factory cup
hope this makes sense
Tim
 

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Hadnt considered that gidyup, but, you might be onto something. I dont think Bridgeview would have actually done the work, they probably would have subbed it out to a shop like RYM (which for the record has always done amazing work with the props ive brought them).

I wonder about the exhaust gas thing, I know some props actually have ports to allow gas to escape at low speed, dont have much experience with them, we tried a set years ago & tore them off very quickly. I dont know that there's a seal is there? I dont know that leg well enough - i think it's just a tolerance fit.

Was thinking about whether you might have bent your shaft as well - depending on how much force was put on the the whole mess. I dont think that would be contributing to the power loss, but, if its bent its a fairly expensive repair ($1k for a regular rotation yamaha 200) - that if not addressed can lead to seal failure in your lower unit which, will wipe your leg out - which is much more costly.

No info on the rpm at WOT from pre-incident and post incident - i think that's critical information to help diagnose.
 

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Discussion Starter #9
Was thinking about whether you might have bent your shaft as well - depending on how much force was put on the the whole mess. I dont think that would be contributing to the power loss, but, if its bent its a fairly expensive repair ($1k for a regular rotation yamaha 200) - that if not addressed can lead to seal failure in your lower unit which, will wipe your leg out - which is much more costly.

No info on the rpm at WOT from pre-incident and post incident - i think that's critical information to help diagnose.
Hello Chewy:

Thanks for the info. In our testing, the RPMs at WOT are pretty much the same before and after the downrigger wire got tangled. My wife and I took the boat out on Tuesday and i was able to run the engine up to 5200rpm. Still, the faster the engine turns the more it seems like we are losing power which fits with the problem being related to exhaust gasses leaking into the prop. I will be hauling the boat out next Tuesday and I have an insurance appraiser coming by to talk with the mechanic and to do their own appraisal. My thinking at this point is that I may need to replace the leg housing. I called Inlet Marine to see if the seal was broken if they could just replace the seal and they said that wasn't possible. I don't know what the exhaust seal looks like but I will find out more next week. Thanks everyone for your input. Dwelo, I hope you are able to get that electrical problem figured out. It sucks not being able to get out on the water especially when the Pinks and Coho are starting to roll in.
 

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Discussion Starter #10
Hello All:

Just a quick update on the issue I am having with my boat. Hauled the boat and had the mechanic take a look. the lower leg housing where the prop slides onto the spindle and into the housing was all chewed up. Had a marine appraiser come down and take a look. They agreed that the lower leg housing was toast. Then the debate turned to how to fix it. Cost was $3500 for new leg housing and about $2000 in labor to take the gears/guts out of the old housing and put it into a new housing. Cost was about $6500 for a whole new lower leg. Insurance decided to try seeing if Inlet could move the gears into the new housing. Inlet Marine is doing the work but has told the insurance company that if they get into it and find anything wrong with the gears then they will replace the entire lower leg 9which i would prefer). Boat has been on the hard for over two weeks now so it is very likely the total costs for replacing the housing will be more than the costs to replace the lower leg. Had no idea you could do so much damage with a downrigger cable. Will be switching my downriggers to braided line this winter.
 
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