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Old 24 August 2018, 10:54   #1
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Humber Destroyer Transom and Engine height

Can anybody help as I'm getting thoroughly confused. I have a new 5.8m destroyer with a transom for a long shaft engine. Humber advised that the engine cav plate should be level with the bottom of the hull so I took some measurements from my old boat and marked up the holes. I thankfully decided to give it one last check before drilling and found a problem. The distance from the top of the transom to level with the bottom of the hull is 545mm (21.5") but the distance from the highest point the transom can sit in the engine to the cav plate is only 508mm (20"), confirmed by measurement and mercury engine drawings.

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If I mount with the cav plate level, the engine saddle will foul the transom. This means the LOWEST I can fit the engine is with the cav plate 37mm above the bottom of the hull. Unfortunately Humber are now closed until Tuesday so I can't ask them.

Does anybody know if this is too high? Am I missing something here but the transom seems very long for a long shaft, and mid way between short and long.

Can anybody with a Humber with long shaft engine measure the cav plate height and transom length and confirm if mine is normal?

Phil
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Old 24 August 2018, 12:18   #2
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Humber Destroyer Transom and Engine height

Phil,
Not a Humber but just measured our XS and transom depth vertically (not on rake) to planing pad is 520mm / 20.5” so about 1” less than yours.
A/v plate on our Merc longshaft is level with planing pad and bolted 2nd hole from top. Top bolts 48mm from top of transom.

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Edit: No idea where middle photo of sib came from - not mine. On phone with sketchy coverage unfortunately.
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Old 24 August 2018, 12:41   #3
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Thanks for those. I'll head back out later with the tape again and do a quick comparison. You seem to have a little space between transom top and engine saddle. Is that the case or is it just how it looks on the photo?

Phil
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Old 24 August 2018, 12:53   #4
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Phil,
Yes quite right, about 22mm between top of transom and saddle.
Hope all goes well with motor installation.
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Old 24 August 2018, 13:10   #5
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My 5.5m Destroyer is approximately 22” transom height for a long shaft engine
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Old 24 August 2018, 13:52   #6
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Quote:
Originally Posted by Phil M View Post
Can anybody help as I'm getting thoroughly confused. I have a new 5.8m destroyer with a transom for a long shaft engine. Humber advised that the engine cav plate should be level with the bottom of the hull so I took some measurements from my old boat and marked up the holes. I thankfully decided to give it one last check before drilling and found a problem. The distance from the top of the transom to level with the bottom of the hull is 545mm (21.5") but the distance from the highest point the transom can sit in the engine to the cav plate is only 508mm (20"), confirmed by measurement and mercury engine drawings.

Attachment 126527

If I mount with the cav plate level, the engine saddle will foul the transom. This means the LOWEST I can fit the engine is with the cav plate 37mm above the bottom of the hull. Unfortunately Humber are now closed until Tuesday so I can't ask them.

Does anybody know if this is too high? Am I missing something here but the transom seems very long for a long shaft, and mid way between short and long.

Can anybody with a Humber with long shaft engine measure the cav plate height and transom length and confirm if mine is normal?

Phil
1 1/2 inch gained by the angle of transom straight edge on keel out transom top down to that an inch above and below is quite common.
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Old 24 August 2018, 14:14   #7
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I'll offer the engine up and see how it looks but sounds like it should be fine. It's just a bit odd as the old boat has a 20" transom and the new is nearly 2" higher but there isn't 2" clear between engine and transom top.

Thanks for the replies though, very useful.

Phil
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Old 29 August 2018, 12:03   #8
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The engine hoist arrived yesterday so managed to offer the engine up to the new transom. I pit a 5mm shim on the transom top and lowered the engine until it contacted the shim, marked the height and drilled the middle holes. On fitting the engine, it is still 35mm too high and there is no scope to lower it without modifying the transom. The transom is made up of an outer plate about 15mm thich which is about 20mm higher than the thicker inner transom and I have about 5mm clearance to the outer plate.

I've contacted Humber and they are looking into it but not got a resolution yet .

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The transom height, perpendicular to the keel (not along the transom) is 21.5". The engine need to drop exactly 2 holes for the keel to be level with the cav plate.

Phil
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Old 29 August 2018, 14:48   #9
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I'm guessing that lip shouldn't be there ideally, strange one. The outboard is probably a little too high but level with keel is probably a little too low.

If that is a solid bit of glass I assume a grinder would fix it if humber is a long travel for you and humber give nod to do that.
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Old 29 August 2018, 15:01   #10
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...The transom height, perpendicular to the keel (not along the transom) is 21.5". The engine need to drop exactly 2 holes for the keel to be level with the cav plate.

Phil
I consider Humber's advice to be good in getting the anti-vent plate level with the planing pad. The issue, as you'll be aware, is ventilation by drawing in air from the water surface, especially problematic in turns. If you use a good prop I think you'll get away with the height you have at the mo - two reasons... the holding of the prop on the water is very dependent on the flow around the lower unit since the prop works in this partially disturbed water and the Mercury lower unit is good in this respect, secondly, your Destroyer has a fairly long and wide planing pad and the water coming off this is also a good laminar flow with no strange vortices as can be the case with a deep V hull.

Take it out and give it a try, though your prop looks a bit general purpose, looking down the blades there's little curvature for hooking on to entrain the water. But worth a try before you start messing with the boat.
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Old 29 August 2018, 15:05   #11
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I'm guessing that lip shouldn't be there ideally, strange one.
Aye it should. It's the joining of the inner transom glass to the outer transom glass. Just Humber's way of doing it.
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Old 29 August 2018, 16:21   #12
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I'll see what Humber say but to lower it will mean cutting transom. I assume Merc are not known for short engines so can only assume it's a taller than average transom. I'd be tempted to trial it before trimming but searching for the correct prop could be expensive and time consuming.

Phil
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Old 29 August 2018, 17:35   #13
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I ran mines on a 22" prop and lifted it up 1 set of holes so there was a gap it never gave any problems with cavitation it was a 5.3 destroyer with a mariner 115 4 stroke on it when I put it together it was offset I think 25mm to the right of the centre line and the holes were lined up and drilled with the engine sitting on the transom it was a long shaft engine it was tried wit several props on it until we put the 22" stainless on it
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Old 29 August 2018, 18:19   #14
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Aye it should. It's the joining of the inner transom glass to the outer transom glass. Just Humber's way of doing it.
I stand corrected, thanks for info.

Probably okay then but worth seeing what humber say tomorrow.
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Old 30 August 2018, 06:39   #15
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I've spoken to Humber who agree it is a little taller than usual but to fully mount the engine as is and see how it goes. If cavitation is an issue, the outer lip can be cut back to drop it a hole.

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Old 30 August 2018, 07:00   #16
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I've spoken to Humber who agree it is a little taller than usual but to fully mount the engine as is and see how it goes. If cavitation is an issue, the outer lip can be cut back to drop it a hole.

Phil
Sounds like a result!

Just for info, cavitation won't be an issue but ventilation might be. Cavitation is a completely different phenomena; it's caused by nucleate boiling of water brought about by extremely low pressure at the surface of the prop. Small bubbles of steam form and when they collapse they release a fair amount of energy, enough to remove particles of metal from the propeller surface. Cavitation can be recognised by a patch of erosion present on the blade surface.

I've no idea how the word cavitation has got into general usage instead of ventilation - perhaps it sounds better to the unknowledgeable.
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Old 30 August 2018, 17:34   #17
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Mounting an engine based on the plate or prop shaft position is quite old school. That said it’s a good starting point.

The ideal way to verify your engine height is, once it is on, run the boat with normal load, normal good cruising speed, and view the plate for position against water. It should be out of the water but getting splashed. If it’s in the water you are too low, if it’s out and bone dry your too high.

I run my propshaft almost level with bottom of the boat. But that’s quite high and on a stepped hull, not for everyone. Try what you have got, before cutting anything off, get a sturdy friend to video the engine and height. It’s easier said than done but a reliable method.

Here is me trying an extra bit of height and a smaller prop. Very light splashing of cav plate, normal cruising boat may be a bit lower

https://youtu.be/UHvq6f1b2I0
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Old 07 January 2019, 13:06   #18
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Yesterday I finally got out for the first time in the humber and am happy to report that the engine height is not too high. I took some photos of the leg showing where the water is coming over the vent plate and am now thinking the engine is perhaps a tad low. Top speed was about 35kts at 5850rpm. Rated speed for the engine is 5000-6000rpm. Holeshot was good and she was on the plane in 5 or so seconds. I'm currently running a stock 17" merc aluminium prop.

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video is here but I can't get it to show

https://vimeo.com/309927366

For reference, this is the leg out of the water. The upper splash plate is the one you can see in the video, not the anti-vent plate.

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So, does this look a little low still? I'm wondering whether upping the engine and changing the prop might make any difference. Perhaps something stainless with more cup and more blades?

Phil
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Old 08 January 2019, 01:50   #19
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if anything from that last pic its a touch high but if your not getting any cavitation and your revs are just about bang on where you would want them id just get back out and enjoy it
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Old 08 January 2019, 02:00   #20
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I would say the engine is to high if your seeing the second plate only . May be an inch or so untill you can see the vent plate and try some turns to see if it is venting
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