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Prop Location, X Dimension

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Prop Location, X Dimension

Old 02-19-2020, 07:29 PM
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Default Prop Location, X Dimension

Is there any pro\con's to how high or low your outdrive sits in the water. I have read that people shorten their lower units, I'm assuming this is to have less in the water? Or is this to have the prop shallower in the water, for some reason, not necessarily just to accomplish less drag?

I'm installing a bravo one on my 24ft deep V fishing boat that previously had an OMC outdrive. I will be glassing the transom up and cutting a new hole for the bravo. This is an offshore fishing boat, I'm not planning on going over 40 mph, maybe mot over 30. I will be doing 200+ mile round trips out to the tuna grounds, often in a four foot swell, sometimes in worse conditions, sometimes better, but I expect it to be smooth enough to run top speed very rarely.
I'm going to have to adjust the height \ placement of the motor, if anyone has experience with a better motor mount than the stock merc mounts I'd love to hear that as well.I will also be running a full hydraulic steering, dual ram setup made by a company called Kool Drive. I have most of the hardware but I still need the brackets that mount to the stern. If anyone has experience with Kool Drive or any contact info I'd appreciate that as well, I couldn't find them online.

Merc recommended that the anti-cavitation plate be level with the bottom of the boat. I'm wondering if there if there is any reason I should go higher or lower.

Thanks

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Old 02-20-2020, 02:18 AM
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For what you are going to use the boat for, I would want the prop fairly deep. I would set it up to have the propshaft about 6" below the hull.
I would use Offshore type fixed mounts for mounting the engine.
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Old 02-20-2020, 02:40 AM
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Thanks Griff, I appreciate the help. I was considering the stock merc mounts that are adjustable in height a little, is that what you mean? I'll figure out how far the shaft will be below the bottom (center) of the hull with the cav plate even with the bottom (center ) of the hull. Do you know what changes with the prop higher or lower?
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Old 02-20-2020, 06:12 AM
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I do even deeper, 7-7.5” below the bottom. Then a simple off the shelf three blade will do everything you need it to.

An offshore mount is one solid piece of steel or AL that actually wraps underneath the motor and has no adjustment once bolted into place. Like a cross member between the two stringers. A bit of a pain in the a$$ to get alignment perfect when installing the motor but might be the better option for the conditions you boat in.
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Old 02-20-2020, 06:58 AM
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If you use the stock mounts, at least fab an aluminum or steel bracket to set them on. Don't mount them directly to the stringer.
Your oil pan, and front accessories will limit how low you can go before hitting the bottom of the boat.

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Old 02-20-2020, 11:55 AM
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Use a front engine mount plate Merc Part# 849143T
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Old 02-20-2020, 06:41 PM
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Thanks guys, that mounting pate is similar to what is in the now\stock. The OMC I'm replacing is about 25 inches from the center of the input shaft (which is actually a ball gear not a shaft) to the center of the prop shaft. Does 22 inches from input shaft center line to prop shaft sound correct for a stock height B1? That's what I measured. The input shaft on the B1 is going to be the same as the crank, correct? I'm assuming that is true on both outdrives.
Currently the center of the crank on the motor is 17 in. above the bottom of the hull. This puts the center line of the OMC prop shaft 8 in below the hull, and the B1 only 5 in. I must be screwing something up..
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Old 02-20-2020, 08:07 PM
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Look up your engine and outdrive combo or closest one on the link below, scroll thru it`s a long list.
i used the dimensions for mounting my engine and it went smoothly.
Also if you choose to use a solid offshore front mount like griff posted, i have that exact one, it is brand new from mercury.
i paid 300 plus shipping.

https://www.mercurymarine.com/en/us/...tion-drawings/
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Last edited by outonsafari; 02-20-2020 at 08:09 PM.
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Old 02-20-2020, 10:50 PM
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Awesome, thanks guys!
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Old 02-21-2020, 01:12 AM
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Originally Posted by tbev View Post
Thanks guys, that mounting pate is similar to what is in the now\stock. The OMC I'm replacing is about 25 inches from the center of the input shaft (which is actually a ball gear not a shaft) to the center of the prop shaft. Does 22 inches from input shaft center line to prop shaft sound correct for a stock height B1? That's what I measured. The input shaft on the B1 is going to be the same as the crank, correct? I'm assuming that is true on both outdrives.
Currently the center of the crank on the motor is 17 in. above the bottom of the hull. This puts the center line of the OMC prop shaft 8 in below the hull, and the B1 only 5 in. I must be screwing something up..
Your measurements are pretty accurate. A 17" X dimension will put a Bravo 1 propshaft about 5" below the bottom. 17-18" X dimensions are pretty common for conventional V bottom single engine performance boats. For how you are going to use the boat. I'd go deeper. You could always leave the engine as is and install a drive spacer to get the prop deeper.
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