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Old 14-03-2010, 07:11   #1
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Proper Prop for 26' Crosby Launch W/Perkins 85 hp

I've got a 26' Crosby launch with a Perkins 85 hp. I use it for harbor tours and just tick it over at 1000rpm. Full throttle is about 2200rpm. The problem appears to be that it came with an under-sized 3 blade prop. I say that because I've driven many launches and they all had great thrust and stopping ability. This one is borderline dangerous if you have to stop quick. Nothing there!! I've called Crosby with less than desireable results. They say all their boats are set up differently and can't/won't help.

Anyone want to give me suggestions on size and pitch? I'd like to go with a four blade.

Thanks. Blair.
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Old 14-03-2010, 07:16   #2
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gear ratio?

What's your gear box ratio?
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Old 14-03-2010, 08:10   #3
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There are a bunch of things that could contribute to this. First off, your hull and prop need to be clear of any marine life. In addition, you need to make sure that the prop gets good water flow, I would doubt that the factory would ship one this way but the hull could have been modified blocking the waterflow. You don't mention the displacement of your boat but if something has been done to make it significantly heavier, that will have this effect. Without knowing exactly what model engine it is, it is hard to say but 2200rpm max sounds low. The two reasons for this are improper gearing/prop and an actual problem with the engine where it isn't reaching full power. Does the engine rev up in neutral to redline?

The engine powerband, gearbox ratio and prop need to be matched to the boat. It is possible that your prop is incorrect. If the prop is too small, it will not get very good grip in the water, someone who is used to seeing this will be able to spot it. If you can go from a stop and redline the engine instantly and then the boat slowly catches up, this is a good possibility. I highly doubt that the prop is too big, it would need to be enormous. You could also have too much pitch which has the effect of putting too much load on the engine which also explains the low maximum rpm. Too little pitch would result in reaching redline rpm but not going very fast.

When you say that you have no stopping ability, is this giving the engine full throttle and what is the rpm? It is actually possible that the prop is overpitched/too large and the load prevents your engine rpm from getting up into the engine's powerband.

If you can find another Crosby just like yours and you can ask the owner what their engine, gearbox, prop combo is, that would be very helpful. Also, prop shops specialize in figuring out this sort of thing and they are really the ones to consult.
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Old 14-03-2010, 09:11   #4
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Quote:
Originally Posted by Ziggy View Post
What's your gear box ratio?
The number is long gone. '88 engine. I'm trying to Google the serial number of engine to try and get gearbox ratio.
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Old 14-03-2010, 09:16   #5
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Quote:
Originally Posted by klem View Post
There are a bunch of things that could contribute to this. First off, your hull and prop need to be clear of any marine life. In addition, you need to make sure that the prop gets good water flow, I would doubt that the factory would ship one this way but the hull could have been modified blocking the waterflow. You don't mention the displacement of your boat but if something has been done to make it significantly heavier, that will have this effect. Without knowing exactly what model engine it is, it is hard to say but 2200rpm max sounds low. The two reasons for this are improper gearing/prop and an actual problem with the engine where it isn't reaching full power. Does the engine rev up in neutral to redline?

The engine powerband, gearbox ratio and prop need to be matched to the boat. It is possible that your prop is incorrect. If the prop is too small, it will not get very good grip in the water, someone who is used to seeing this will be able to spot it. If you can go from a stop and redline the engine instantly and then the boat slowly catches up, this is a good possibility. I highly doubt that the prop is too big, it would need to be enormous. You could also have too much pitch which has the effect of putting too much load on the engine which also explains the low maximum rpm. Too little pitch would result in reaching redline rpm but not going very fast.

When you say that you have no stopping ability, is this giving the engine full throttle and what is the rpm? It is actually possible that the prop is overpitched/too large and the load prevents your engine rpm from getting up into the engine's powerband.

If you can find another Crosby just like yours and you can ask the owner what their engine, gearbox, prop combo is, that would be very helpful. Also, prop shops specialize in figuring out this sort of thing and they are really the ones to consult.
Thanks for your response. I bought used and think I got a bogus prop cuz the owner wanted the original for his second boat. Everything is in order except the prop. Seems tiny compared to other launches i've operated. Cheers.
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