Slightly different outboard - transom question

[M-S-D]

Hi knowledgeable forum. This may have been asked before but couldn't spot it quickly.

Believe I understand the concept of the captivation plate to be level with the bottom of the transom.

Having a scout around the hard standing, all the boats with outboards have a hull which resembles a flattened "V", i.e .the lowest point of the hull is lined up with where the outboard would be.

Now, for our sins, we are converting an old Dawncraft that in the past had an inboard engine, and the hull here is not the "V" style, but is "cut out". i.e. about 1.5 feet either side of centre is the lowest part of the hull, and lined up with where the outboard would be is about 4 inches higher up.

So my question is, should we be taking the measurement from the lowest point of the hull which is to the side, or from the centre which is higher up?

Additional info, this is just for pottering about on inland water ways. Also intention is to add an "outboard pod" so the outboard will actually sit say 16 or so inches behind the rear of the boat.

We probably have enough "play" to go for a long shaft or an extra long shaft, and advice here may clinch it.

Just as an aside, it seems to me from measuring the actual outboards, that the captivation plates for say a 20" long shaft actually seem to be more like 22 to 23 inches. Maybe this is just manufacturers building in some extra to allow for angle etc.

Thanks in advance.

 

[Tranona]

The prime measurement is to the waterline and the secondary is to check the cavitation plate is roughly where the bottom of the hull is otherwise you will not get clear water to the propeller.

 

[VicS]

Hi knowledgeable forum. This may have been asked before but couldn't spot it quickly.

Believe I understand the concept of the captivation plate to be level with the bottom of the transom.

Having a scout around the hard standing, all the boats with outboards have a hull which resembles a flattened "V", i.e .the lowest point of the hull is lined up with where the outboard would be.

Now, for our sins, we are converting an old Dawncraft that in the past had an inboard engine, and the hull here is not the "V" style, but is "cut out". i.e. about 1.5 feet either side of centre is the lowest part of the hull, and lined up with where the outboard would be is about 4 inches higher up.

So my question is, should we be taking the measurement from the lowest point of the hull which is to the side, or from the centre which is higher up?

Additional info, this is just for pottering about on inland water ways. Also intention is to add an "outboard pod" so the outboard will actually sit say 16 or so inches behind the rear of the boat.

We probably have enough "play" to go for a long shaft or an extra long shaft, and advice here may clinch it.

Just as an aside, it seems to me from measuring the actual outboards, that the captivation plates for say a 20" long shaft actually seem to be more like 22 to 23 inches. Maybe this is just manufacturers building in some extra to allow for angle etc.

Thanks in advance.

Strictly speaking the plate is an "anti ventilation" plate. Its purpose is to prevent air being drawn down into the prop. Cavitation is something quite different, but there is a good video at https://www.youtube.com/watch?v=ON_irzFAU9c which explains it.

As Tranona says the "shaft length" should equal the height of the transom , or engine mounting bracket, top above the water level. This will put the AV plate a little below the water surface and approx in line with the bottom of the transom ahead of the engine. It should not be significantly above the bottom of the transom but may be an inch or so below it.

For pottering about on inland water ways the exact height is not going to be critical just so long a the engine is mounted so that the water pump is submerged.

 

[Seajet]

In less pompous and more useful terms, the prop' and cooling intake need to be in the water - easier with an engine well than an engine perched on a transom - and the shaft length needs to be short enough to stow in a locker if possible while sailing, otherwise it's weight in the wrong place - and can you reach the controls in close manouvering, or have remotes rather than having to look behind at the critical moment ?!

It will be important to be able to raise the leg to reduce drag for cruising sailing speed, also watch out when alongside walls rafted out, when other people's shore lines may go around the engine and break off bits.

 

[VicS]

In less pompous and more useful terms, the prop' and cooling intake need to be in the water - easier with an engine well than an engine perched on a transom - and the shaft length needs to be short enough to stow in a locker.

For instance the outboard well is just to port on the Anderson 22, using a standard shaft engine; in 40 years I have only had the prop' break the surface once, in overfalls at the Liitle Russel channel, Guernsey.

It's important to stow the outboard ( one lifts it and gives the coolant water say 30 seconds to drain ) then fit the well fairing plug, which gives well over a knot advantage in sailing speed.

All this is completely irrelevant to someone with an old river cruiser on inland waterways.

 

[Seajet]

Which is why I adapted my reply; do keep up !

 

[VicS]

Which is why I adapted my reply; do keep up !

Funny why your "adapted" reply does not appear in my quote then

 

[William_H]

The rules on o/b motor depth the plate being level with the bottom of the hull is aimed at a hull that is planing where the bottom of the hull/transom is at water level. Hull is on top of the water. Not sure where water line would be on your hull when planing but not a real concern Under these conditions you don't want the plate or water intake higher than water level.
For non planing conditions you want the plate low enough to cope with pitching either in waves or with weight (people) forward. For these conditions deeper the better however a deeper motor provides more drag from the shaft etc so is somewhat more efficient up higher providing plate is always below water level. Hence many sail boats have an adjustable height bracket. But this is not necessary with your flat bottomed straight keel design where the bottom of the transom will be below water level when going slow. Sail boats tend to have a banana shaped profile from the side where the bottom curves up to near water level at the transom hence more inclined to pitch. olewill

 

[Tranona]

Which is why I adapted my reply; do keep up !

Not sure you have bothered to read the original question as you your first response was completely useless. Your edited reply is not really "adapted" - all you have done is remove some of the more useless information. What is left is still of little help to the OP.