Transom Jack Plates

Adj · 19 October 2021, 21:12

Anyone had any experience lowering an engine by means of a Jack Plate? I've a long shaft (21") Evinrude 50 on a 25" Transom. (New purchase - bought a 'project boat', so second hand and I never realised the engine was too high). Apparently I need to drop the engine by 4" to stop cavitation problems and get the engine to work properly. I am loath to cut the transom as the 'back-wash' on throttling off nearly came over the top as it is.

Someone suggested a Jack Plate, but when I went to buy one I was told that they don't work with lowering an engine, only lifting one up, as the steering bar etc will get caught and the engine will not tilt back without catching the transom.

Has anyone tried this or have any insight? Thanks

Limecc · 19 October 2021, 23:38

Last advice you were given seems fair.
Where's your AV plate in relation to the bottom of the keel? Actual measurement?

May be possible to nibble away at the differential. Inch or two off the transom, smaller dia prop, fit hydrofoil etc might just give what's needed to reduce your cavitation ventilation.

jonp · 19 October 2021, 23:44

Would love to see the antivenelation plate on the engine when its trimmed down to see how high it must be to cavitate. Is the engine already mounted on its lowest hole ? if so how does it sit while on the water in a following sea ?
What prop is on it and have you looked at getting some cupping done to the prop, which might improve the issue. Also a 4 blade might improve grip and holding. I tend to change to 4 blade props on most of my boats as I find they often improve several characteristics of the boat for where I live.

I have a few spare props and also work with an outboard mechanic at times who allows me to try out any of his props. I was recently quite surprised after testing 6 props on my current boat that a relatively cheap 4 blade aluminium prop from 'Turning point" out performed three stainless props that all got the boat into its max rpm range. Two 13x17 stainless props from different manufacturers had a difference of 500rpm and one of them constantly caused cavitation and had very poor grip.

Something many people don't consider with jacking plates is the extra amount of force applied to the transom for every inch from the boat an engine is placed (Google search those weight formulas). If your boat has been designed to handle extra force from extending the distance of the engine then that might very well improve things.

With jacking plates, the further from the boat "4"6"10"the engine is placed the higher it will need to be placed, so even a 4 inch plate will sit the prop deeper in the water even if mounted at the same height as it was on the boat.

Adj · 20 October 2021, 07:08

That's for taking the time to respond, chaps. I'll post some photos when I get back from work - that'll be more clear...

Matt · 20 October 2021, 08:00

Yeah, pics will help. Possibly a stainless prop with some cup in would help resolve cavitation - but if it's too high, you might still not be able to get enough up trim anyway to carry the bow. Lots of variables.

Adj · 20 October 2021, 18:57

Sorry guys - unable to upload any photos. Keeps saying they are invalid even though I've shrunk them down to 12KB, which is likely not to be useful!

So imagination is going to be the way forward!

When I place a yard stick on the fin of the engine, it comes to 4" above the 'V' of the bottom of the keel. The transom looks like it has been raised 4" previously and there are two 5mm steal plates bolted on both sides and shaped to go over the top.

The symptoms are: 1) When in a turn it feels like if driving a car you've suddenly put your foot on the clutch - engine just revs and there is no forward drive.
2) Same happens if I go over another boats wake;
3) The engine struggles to sit at 4500 to 5000 RPM, especially with more than one person in the boat, and the slightest bit more throttle, and she spins up to 6000 RPM. From reading other posts, it feels like she cannot 'bite' the water and keeps slipping.
4) 3 hours boating trashed 22L of fuel at 4k - 5k RPM.
5) Unable to get more than 22 knots. And with 3 people in, that drops to 18 knots.

The engine definitely needs lowering, but I do not want to remove the steal plates and cut her down. Don't know what I will find under there. And then if the 50 Evinrude proves to be too weak, then I have wasted the effort. So a Jack Plate seemed like a logical solution as a test-drive, but then I was told they would not work in lowering the engine.

So I am looking at having to buy an extra-long Yammy 70 unless anyone has an idea about the Jack Plate.

Thanks guys

Limecc · 20 October 2021, 20:40

Don't be afraid of looking at the transom. It's the most important part of the boat so if there's rot then you need to know about it!

spartacus · 21 October 2021, 07:19

Don't cut the transom, that's a one way street. The hydraulic jack plates I've seen are serious bits of kit especially in the US, and aren't cheap.

Change the engine is my advice. https://www.findafishingboat.com/mar...roke/ad-119816

beamishken · 21 October 2021, 08:38

Doesn't sound like the boat was built for an extra long engine, sounds like its been jacked up from a longshaft for some reason, I'd strip off the added bits & see whats underneath if its weak it needs fixing anyway, better to find out before something bad happens. You dont say what size boat it is but presumably fairly small & very few small boats have or need xl transoms & just as unusual to find small hp xl engines so best option is sort the transom so it matches the engine

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Limecc · 21 October 2021, 09:24

ADJ - Your photos need to have .jpg or .png file extensions so they are probably in the wrong format rather than the wrong size. I think the forum automatically resizes anyway. Can a friend help you?

Matt · 21 October 2021, 10:14

Also - if you try and upload a file with filenames created by an iPhone, I find they often won't unload. So you might need to rename them so a shorter file name name.

And iphones often default to this "heic" format these days which the forum doesn't handle - so you need to use paint or something to save as jpg or png like Limecc says.

Matt · 21 October 2021, 13:51

WRT to the issues on this boat.

Transom height & therefore leg length are key design criteria on a boat. It wouldn't be so hard, for example, to have, rather than a transom jack, a standoff box fabricated (a fixed affair). But then you need to start consider the effect of moving the CG back, plus lowering the motor so much - is it too low then (so the cowl ends up bit too much in the water as you come off the plane), and will the steering & other connections start to foul - which might mean you need more standoff.

You might be able to make the current setup work a little better with a stainless prop with a little more diameter, 4 blades rather then 3, a bit of cup in the blades.....but this type of direction is where you go for higher performance stuff.....my boat probably runs about the same propshaft height you're dealing with here - but it's by design and I've designed the setup it around that, acknowledging the compromises it'll come with.

Adj · 21 October 2021, 19:56

Was me being dozy! Macs call the .jpg files .jpeg, so .png worked

Adj · 21 October 2021, 19:58

It is a 5.2m rib. The one photo is with a rules balanced on the fin. Sorry about the lighting - had to use the car headlights. But from what some of you have said, I am leaning more towards a bank loan and new engine.....

jeffstevens763@g · 21 October 2021, 20:17

Has that plate on the transom been put on to suit a 25 inch engine if it was took off my guess is that the transom level drops. If it were me I would drill a small hole through to see if it’s solid wood where the engine sits in the 4 inches down from the top

beamishken · 21 October 2021, 21:45

Looks like your looking for a bodge to compensate for a bodge. Best option is strip the previous bodge off, return the transom to the correct longshaft height & your sorted, why would you consider fitting a new engine to a cobbled together transom? Remove the plates confirm the original transom is good or not & either fix it or dont fix it as appropriate. A 5.2m rib doesn't need an xl transom, if your in danger of flooding as you come off the plane either come off gradually or turn 90 degrees before the stern wave hits.
No need for stand off brackets or jack plates just fix what you have properly & problem solved

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Matt · 21 October 2021, 22:35

I don't disagree with this.

A stainless prop would also help - maybe a vengeance or even a laser 2. But it's guess work. For all around general use, you probably want it to be at least a couple of inches lower.

It's not necessary to have the transom plate flush with the bottom - heck, my propshaft is about the same height as yours (but I designed it that way), but for a good all-rounder, it does want to be a little lower IMVHO.

spartacus · 22 October 2021, 06:28

Got any pictures of the transom so we can see?

Also what RIB is this? Even with changes to get anti cavitation plate in line with keel, a 50hp isn't going to set the heather alight, but at least you should see fuel economy return to some reasonable burn rate.

beamishken · 22 October 2021, 06:46

Quote:

Originally Posted by Matt

I don't disagree with this.

A stainless prop would also help - maybe a vengeance or even a laser 2. But it's guess work. For all around general use, you probably want it to be at least a couple of inches lower.

It's not necessary to have the transom plate flush with the bottom - heck, my propshaft is about the same height as yours (but I designed it that way), but for a good all-rounder, it does want to be a little lower IMVHO.

Av plate Probably 1" above the keel line would be fine for low power like this so if he takes off the 4" bodge up and bolts it 1" above the original transom its problem solved.
The issue seems to be deep down he probably knows the transom is rotted & is frightened to look & get it repaired properly. We all know patched transoms like this dont heal up & the best option is a proper structural repair instead of the workaround that hes looking for.

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Limecc · 22 October 2021, 10:14

Repairing a transom need not be a trial, it's all about having the right mindset and repair technique. Plenty of advice here and videos (good and bad, learn from both) on YouTube.

BeamishKen's spot on. When it's done you've added resale value, avoided possible disaster at sea and the effort of a longer shaft engine, bodging a bodge.