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Author Topic:   Motor Position on Transom

weed monkey posted 03-17-2002 05:39 PM ET (US)
First of all, thank you all for the great information you share. Yesterday, I dropped my 1969 Nauset off in Ashland VA to have a new 90 Yamaha installed. I read the threads for the last 2 years on the splashwell challenge. However, I am wondering how do you determine or what is the correct height to mount the motor. Is there a relationship to the cavitation plate and the bottom of the boat?
I notice there are many diehard Mercury fans and I hope I have made the right engine choice. I have been powered by Mercs since they came with green engine covers (with the exception of the Fisher that was on the bost when I bought it).
Thank you again,
Bob
SpeedyWhaler posted 03-17-2002 07:11 PM ET (US)
Usually the cavatation plate is even with the bottom of the hull.
kingfish posted 03-17-2002 10:47 PM ET (US)
Weed monkey and Speedy Whaler-
Actually, if the motor is mounted on the transom (as opposed to being mounted on a bracket so the motor is held back from the transom), the motor is likely to be in best performance range when the cavitation plate is 1" to 1 1/2" above the lowest point on the transom. Gauge it by holding a straight edge along the lowermost part of the bottom, in line with the length of the boat. Extend that line back to the lower unit and measure from there.
kingfish
ffk posted 03-19-2002 09:41 PM ET (US)
Kingfish
I am trying to get the height of the engines on my '72 Outrage 21 worked out, perhaps you could offer your thoughts. Twin 90hp Yamaha currently installed in the 2nd hole which places the cavitation plate centerline at the intersection of the hull bottom and transom. When the boat is up on plane, the cavitation plate is running about 1-1 1/2 inches below the water surface. My understanding is that the plate should be just at or slightly above the waters surface. I was going to move the engines up 2 holes in hopes of not having to do the job twice. What issues will surface if I am slightly higher while running. I appreciate any input from the forum.
Thanks, Frank
Bigshot posted 03-20-2002 09:55 AM ET (US)
You'll know when it is too high. It will resonate at cruise like the prop is out of the water slightly(it is) and you will probably cavitate on holeshot. I like mine so high that when you nail it from a stop it goes and hits a high RPM say 5700 then it slips into 2nd gear(actually stops slipping)and you are gone. This is where I get the best performance. High enough that she slips on takeoff, believe it or not is the same effect as vent holes on a prop or a stall converter on a car, which make her scream.
If you do a gradual holeshot you will not tell the difference from a lower engine. make sure hull is clean, the slightest scum will cause cavitation.
Summary...raise her all the way up and keep coming down a hole until happy. If she handles great on top hole, you could probably go higher. Usually the engine will be about 1.5 inches off the transom. You should be able to see the PTnT rod.
lhg posted 03-20-2002 06:11 PM ET (US)
You can always tell when a boat is running with the anti-cavitation plate too low, a real speed killer. The motor throws a distinct "V" shaped spray pattern as the water flow hits above the anti-cavitation plate. With engines properly mounted, this spray pattern does not exist. Looking over the transom on plane, you should clearly be able to see the top surface of this plate, with no, or little, water flowing over it.
kingfish posted 03-20-2002 09:51 PM ET (US)
Frank-
What they said---
My guess is that if you are currently running with your cavitation plate 1" to 1 1/2" below the surface, you could go 2 holes and be OK, if your purpose is to make the adjustment once and not mess with it again for a while.
kingfish
kingfish posted 03-20-2002 09:51 PM ET (US)
Frank-
What they said---
My guess is that if you are currently running with your cavitation plate 1" to 1 1/2" below the surface, you could go 2 holes and be OK, if your purpose is to make the adjustment once and not mess with it again for a while.
kingfish
kingfish posted 03-21-2002 09:32 AM ET (US)
Hmmm - is there an echo in here?
Whaletosh posted 03-21-2002 10:55 AM ET (US)
ffk,
If you are going to take you motors off you might want to see if a manual jack plate for them is worthwhile. Cook Manufacturing (and others) makes one that is $200 apiece. this would allow you to rise and lower the motors with a prop wrench. The cook units are beefy; others have made positive comments about Bob's Machine shop units as well.
ffk posted 03-21-2002 09:17 PM ET (US)
Thanks for the replies. It seems that we are all on the same page. I just needed a little assurance. The engines are currently sitting 1.5" above the transon. When I raise them up with the 2 remaining holes there will be a 3" space. I may fabricate a removable filler panel to close up the gap for cosmetic reasons as well as keeping more water out when backing down.
The subject of the jack plates is something that I have been tossing around myself, trying to figure out if they would be a good investment. From what I can gather, the setback resulting from the installation will be beneficial in the way of getting the props into cleaner water. This is a '72 with the transoom actually set forward about 8". With the exception of the initial height adjustment convienience, when else would the adjustibility of the jackplates be useful?
Thanks again, Frank
Bigshot posted 03-22-2002 10:54 AM ET (US)
When you want to go up or down hahahah!
Actually once set, lock it and leave it.

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Author	Topic: Motor Position on Transom
weed monkey	posted 03-17-2002 05:39 PM ET (US) First of all, thank you all for the great information you share. Yesterday, I dropped my 1969 Nauset off in Ashland VA to have a new 90 Yamaha installed. I read the threads for the last 2 years on the splashwell challenge. However, I am wondering how do you determine or what is the correct height to mount the motor. Is there a relationship to the cavitation plate and the bottom of the boat? I notice there are many diehard Mercury fans and I hope I have made the right engine choice. I have been powered by Mercs since they came with green engine covers (with the exception of the Fisher that was on the bost when I bought it). Thank you again, Bob
SpeedyWhaler	posted 03-17-2002 07:11 PM ET (US) Usually the cavatation plate is even with the bottom of the hull.
kingfish	posted 03-17-2002 10:47 PM ET (US) Weed monkey and Speedy Whaler- Actually, if the motor is mounted on the transom (as opposed to being mounted on a bracket so the motor is held back from the transom), the motor is likely to be in best performance range when the cavitation plate is 1" to 1 1/2" above the lowest point on the transom. Gauge it by holding a straight edge along the lowermost part of the bottom, in line with the length of the boat. Extend that line back to the lower unit and measure from there. kingfish
ffk	posted 03-19-2002 09:41 PM ET (US) Kingfish I am trying to get the height of the engines on my '72 Outrage 21 worked out, perhaps you could offer your thoughts. Twin 90hp Yamaha currently installed in the 2nd hole which places the cavitation plate centerline at the intersection of the hull bottom and transom. When the boat is up on plane, the cavitation plate is running about 1-1 1/2 inches below the water surface. My understanding is that the plate should be just at or slightly above the waters surface. I was going to move the engines up 2 holes in hopes of not having to do the job twice. What issues will surface if I am slightly higher while running. I appreciate any input from the forum. Thanks, Frank
Bigshot	posted 03-20-2002 09:55 AM ET (US) You'll know when it is too high. It will resonate at cruise like the prop is out of the water slightly(it is) and you will probably cavitate on holeshot. I like mine so high that when you nail it from a stop it goes and hits a high RPM say 5700 then it slips into 2nd gear(actually stops slipping)and you are gone. This is where I get the best performance. High enough that she slips on takeoff, believe it or not is the same effect as vent holes on a prop or a stall converter on a car, which make her scream. If you do a gradual holeshot you will not tell the difference from a lower engine. make sure hull is clean, the slightest scum will cause cavitation. Summary...raise her all the way up and keep coming down a hole until happy. If she handles great on top hole, you could probably go higher. Usually the engine will be about 1.5 inches off the transom. You should be able to see the PTnT rod.
lhg	posted 03-20-2002 06:11 PM ET (US) You can always tell when a boat is running with the anti-cavitation plate too low, a real speed killer. The motor throws a distinct "V" shaped spray pattern as the water flow hits above the anti-cavitation plate. With engines properly mounted, this spray pattern does not exist. Looking over the transom on plane, you should clearly be able to see the top surface of this plate, with no, or little, water flowing over it.
kingfish	posted 03-20-2002 09:51 PM ET (US) Frank- What they said--- My guess is that if you are currently running with your cavitation plate 1" to 1 1/2" below the surface, you could go 2 holes and be OK, if your purpose is to make the adjustment once and not mess with it again for a while. kingfish
kingfish	posted 03-20-2002 09:51 PM ET (US) Frank- What they said--- My guess is that if you are currently running with your cavitation plate 1" to 1 1/2" below the surface, you could go 2 holes and be OK, if your purpose is to make the adjustment once and not mess with it again for a while. kingfish
kingfish	posted 03-21-2002 09:32 AM ET (US) Hmmm - is there an echo in here?
Whaletosh	posted 03-21-2002 10:55 AM ET (US) ffk, If you are going to take you motors off you might want to see if a manual jack plate for them is worthwhile. Cook Manufacturing (and others) makes one that is $200 apiece. this would allow you to rise and lower the motors with a prop wrench. The cook units are beefy; others have made positive comments about Bob's Machine shop units as well.
ffk	posted 03-21-2002 09:17 PM ET (US) Thanks for the replies. It seems that we are all on the same page. I just needed a little assurance. The engines are currently sitting 1.5" above the transon. When I raise them up with the 2 remaining holes there will be a 3" space. I may fabricate a removable filler panel to close up the gap for cosmetic reasons as well as keeping more water out when backing down. The subject of the jack plates is something that I have been tossing around myself, trying to figure out if they would be a good investment. From what I can gather, the setback resulting from the installation will be beneficial in the way of getting the props into cleaner water. This is a '72 with the transoom actually set forward about 8". With the exception of the initial height adjustment convienience, when else would the adjustibility of the jackplates be useful? Thanks again, Frank
Bigshot	posted 03-22-2002 10:54 AM ET (US) When you want to go up or down hahahah! Actually once set, lock it and leave it.