Captivation Plate 3" lower than Transom, how big of an issue

[diveman05]

have a 14' Tri hull that i just got a newer motor for due to the other one being all sorts of problems. Has a 33 HP engine and the boat weighs approx 800 lbs. We've taken it out a few times and it will got 16-18mph, but it doesn't seem very happy doing it and there is some spray from below the engine. I did some research and measured my captivation plate from the bottom of the hull and it is about 3" lower than the hull, in terms of performance is it worth going through the work to raise this 2-3" so its even or at least close?

 

[JB]

Re: Captivation Plate 3" lower than Transom, how big of an issue

That is correctly called the anti-ventilation plate, diveman. Yes, it is slowing your boat and producing excess spray. It should be level with, or even slightly above, the bottom of your boat.

It would be worth the effort to me, even if I had to use a jackplate to get there.

 

[diveman05]

Re: Captivation Plate 3" lower than Transom, how big of an issue

That is correctly called the anti-ventilation plate, diveman. Yes, it is slowing your boat and producing excess spray. It should be level with, or even slightly above, the bottom of your boat.

It would be worth the effort to me, even if I had to use a jackplate to get there.

JB thanks for the reply, thankfully the motor is light so it at least won't be to back breaking! I appreciate your quick response!

 

[kfa4303]

Re: Captivation Plate 3" lower than Transom, how big of an issue

+1. I used to have an old '66 33 hp myself. The old OMC motors used to have an 18" leg length which is about 3" longer than a modern 15" "short shaft", but about 2" shorter than a modern "long shaft". As a result, they usually have to be raised a few inches. I actually just made a transom riser for my '66 20 hp and it works great. Do a search here in the forums as well as at sites like tinboats.net and microskiff.com for tons of cool DIY jackplates and transom risers. Once in stalled, you should notice lighter steering, shallower draft, quicker hole shot and better overall performance and fuel economy. Keep us posted.

 

[jimmbo]

Re: Captivation Plate 3" lower than Transom, how big of an issue

I think the reason the OLD OMCs had longer than standard shaft lengths was, in the 50s, 60s and on some cases, into the 70s, these engines had a second cooling water inlet above the antiventilation plate. If these engines were to be mounted withthe antivent plate flush with the bottom of the boat there was probably a good chance the waterpump might just end up trying to suck up air once the bpoat was planing.

 

[kfa4303]

Re: Captivation Plate 3" lower than Transom, how big of an issue

Yes and no. The primary reason for the odd leg length was that transom sizes were not yet standardized (15" regular-20" long), as a result OMC see to have sort of "split the difference" at 18". While it is true that they do have a water uptake above the anti-plate, it really only comes into play at low speeds/idle when the motor is sitting relatively low in the water. The pumps themselves are relatively weak and are not self priming, hence the need to use a test tank. However, once the boat is at speed and on plane water is forced into the screened opening directly in front of the prop then onto the water pump and up to the powerhead. At this point, the water uptake found above the anti-vent plate can/should break the surface of the water. This will reduce drag while still allowing plenty of water to be taken up by the motor. As the boat slows down again, it will again sit lower in the water and resubmerge the uptakes above the anti-vent plate. Not to worry, jackplates/transom risers have been used on these old motors for years. The old timey racers would even use shims as little as 1/4" to dial in just that right height. Obviously, you don't want to set the motor up too high and it will take some experimentation to get it dialed, but it's well worth the effort. Your boat will steer much more easily, get on plan faster and have a shallower draft. I also stand up using a tiller extension most of the time while underway, and having the motor a few inches higher reduces the angle of the tiller a bit which makes the throttle a bit more responsive too. If you want a fancy-shmancy jackplate, check out bobsmachineshop.com.

 

[jimmbo]

Re: Captivation Plate 3" lower than Transom, how big of an issue

Yes and no. The primary reason for the odd leg length was that transom sizes were not yet standardized (15" regular-20" long)

I am looking in a 1958 35hp johnson owners manual. It clearly gives the boat transom height demension as 15" or 20". Back in the 50s boats often had a definite keel down the center of the boat. Maybe that's why OMC buried their engines deeper. If I were raising one of the older OMCs, I would be blocking off the upper water inlet.

 

[diveman05]

Re: Captivation Plate 3" lower than Transom, how big of an issue

Yes and no. The primary reason for the odd leg length was that transom sizes were not yet standardized (15" regular-20" long), as a result OMC see to have sort of "split the difference" at 18". While it is true that they do have a water uptake above the anti-plate, it really only comes into play at low speeds/idle when the motor is sitting relatively low in the water. The pumps themselves are relatively weak and are not self priming, hence the need to use a test tank. However, once the boat is at speed and on plane water is forced into the screened opening directly in front of the prop then onto the water pump and up to the powerhead. At this point, the water uptake found above the anti-vent plate can/should break the surface of the water. This will reduce drag while still allowing plenty of water to be taken up by the motor. As the boat slows down again, it will again sit lower in the water and resubmerge the uptakes above the anti-vent plate. Not to worry, jackplates/transom risers have been used on these old motors for years. The old timey racers would even use shims as little as 1/4" to dial in just that right height. Obviously, you don't want to set the motor up too high and it will take some experimentation to get it dialed, but it's well worth the effort. Your boat will steer much more easily, get on plan faster and have a shallower draft. I also stand up using a tiller extension most of the time while underway, and having the motor a few inches higher reduces the angle of the tiller a bit which makes the throttle a bit more responsive too. If you want a fancy-shmancy jackplate, check out bobsmachineshop.com.

I was checking on my motor this weekend and i'll post pictures when i get home but i found no water intake below the plate only the one above it that you've mentioned. Below the plate it all solid except bolts and one little what looks like drain hole about 1/16" big. Its a 1970 Johnson, 33EL70M model, you don't happen to know if I'm missing something obvious, which is very possible for me

 

[kfa4303]

Re: Captivation Plate 3" lower than Transom, how big of an issue

Yeah, it's kind of hard to see. Here's a pic of each of the water uptakes. Again, the small uptakes above the anti-vent plate are only in use a low speed/idle, when the motor is well submerged. However, as the boat picks up speed and goes on plane, the anti-vent plate should just barely break the surface of the water at which point the forward motion of the craft and the prop are forcing water under high pressure into the large, screened intake found just in front of the prop itself. Here some pics. Be sure that the screen in nice and clean as it has a tendency to get funky over the years. There's really no way to remove it, but a bit of carb cleaner and a wire brush will usually do the trick. Go back and look at some vintage race photos from the 50's-60's and you'll see guys using jackplates/transom risers/shims, etc...all over the place. Everything old is new again, I suppose. They're particularly popular down here in FL, LA, TX where folks are always trying to get into skinnier and skinnier water (less than 6", yeah inches not feet). Here are some handy pics.

 

[diveman05]

Re: Captivation Plate 3" lower than Transom, how big of an issue

Yeah, it's kind of hard to see. Here's a pic of each of the water uptakes. Again, the small uptakes above the anti-vent plate are only in use a low speed/idle, when the motor is well submerged. However, as the boat picks up speed and goes on plane, the anti-vent plate should just barely break the surface of the water at which point the forward motion of the craft and the prop are forcing water under high pressure into the large, screened intake found just in front of the prop itself. Here some pics. Be sure that the screen in nice and clean as it has a tendency to get funky over the years. There's really no way to remove it, but a bit of carb cleaner and a wire brush will usually do the trick. Go back and look at some vintage race photos from the 50's-60's and you'll see guys using jackplates/transom risers/shims, etc...all over the place. Everything old is new again, I suppose. They're particularly popular down here in FL, LA, TX where folks are always trying to get into skinnier and skinnier water (less than 6", yeah inches not feet). Here are some handy pics.

View attachment 162671View attachment 162672

Great diagram, that should be a sticky somewhere for people like me! Anyway thank you again, i am working on a DIY plate right now off some plans i found. and 6" of water, that is crazy! i get nervous in my old boat in like 2 feet of water, maybe thats because my motor drags so low though

 

[Chinewalker]

Re: Captivation Plate 3" lower than Transom, how big of an issue

We used to run a Johnrude Big Twin 30 short shaft at about 19-inches on the transom of a Sid-Craft Hornet utility racer, and it pumped lots of water at speed...

 

[kfa4303]

Re: Captivation Plate 3" lower than Transom, how big of an issue

so that's why they call you chinewalker Yikers! That's a lot of motor on a little boat. yeah, I'm not real big on running quite so skinny, either. Too much risk to the grass flats and boat, but lots of folks do it. Check out mickroskiff.com for all sorts of crazy tunnel hulls, poling skiffs, flats boats etc... designed for getting into skinny water. Here's a pic of the sort of thing you need. She's kind of a one trick pony, but she's fast as bullet, light as a feather and purty as can be. Her name is "La Flaca" (skinny lady) and was made in a guys garage!!!!

La Flaca:





Random poling skiffs that are popular down here: