Porpoising

Hi guys new member
I have a regal empress and have Repowererd with a pod and 225 Honda I built a pod 2 nd one. And my boat porpoise out of control at speed I made it from alloy and followed the hull and keel lines stepped up about 2 inches from keel any body got any ideas how I can put this right

 

You shifted the center of weight of the boat. From the description, seems like you also lengthen the hull with the pod. The line of thrust has to go, roughly, through the center of weight of the boat. The geometry is shifted, and may not be possible to fix it. Basically, you screwed up a good design. It may be possible to lessen the problem with large trim tabs.

 

Hi gonzo I know I have screwed up lol I did try and basically follow the shape of the hull with a step up it originally had a mercrusier in it what was a disaster v6and V8 I see all your boats across the pond see lots out on pods motor hanging off the back it is extended 24 in off the back so am trying to understand how some how can put this right my first pod was just a squarish type box didn't give me this problem but it took longer to get on plane were this one jumps out the water like scalded cat but at speed starts going wrong if I have more wieght at the stern seems to make the matter worse I have seen two types of pod ones that seem to follow hull shape and ones with big angle up towards the motor on a skinny box with tread plate its long shaft so couldn't get that sort of angle 24 in of the transom If I lower the pod to level with hull would that make it any better with 10 to 20 dog incline to motor

 

Any ideas how can post any pics

 

What happens when the engine is trimmed right down ? Still porpoises ? It may be your pod had insufficient rake, if so you could fit transom wedge to get more tuck-under.

 

Think I set the rake 15 degrees looks like it has a lot of tuck under when first built the pod it did work with a couple inch trimmed up then I was carrying a bit of weight at the stern 200 kg and could not get it to trim right I went and collected my mooring chains as it lives on a tidal eustary 6 months year when I'm out fishing its close season now so trying to improve boat handling while its trailered the motor is lifted 2 inch up from keel matching the pod line also I tow a 1 ton rib then it trims fine but without the tender very porpoisey Think a wedge may help I built the pod nearly the same width as the transom to offset the weight of the motor also when towing its attached to the bow of the towing boat so that helps it being pulled down 95 % I tow on starboard side. Not behind I was thinking of wedging the pod up a bit and wedging the motor down Also the more barnacles it collected on the pod the worse it got Hope all this helps any more info I can give no problem

 

I never felt like the bow was riding to high trimmed up to much

 

What degrees rake should I have on the transom for a Honda 225?

 

Rake of 16-17 degrees would be about right.

 

Does anyone think that the step up of the pod 2 in from keel line or would it be better without a step up on a 24 in from transom plate

 

I'd think 2 inches would be about right. Possibly could go up another hole even.

 

Yep that what I thought I'll transom wedge it give it bit more tuck under can always trim out but only can trim into rake angle

 

Basically, you need to move or add weight as far forward as possible and reduce the amount of motor trim. One of the odder things that happen when you run the prop off axis is that most will actually produce more thrust the greater the angle of skew. So as the boat pitches up, the prop bites harder (and the force parallel to the surface increases, in spite of the angle), making the problem worse. As the nose pitches down, the prob bites less, making the problem worse. The amount that this happens depends on the relative slip of the prop, and on it's P/D ratio, but it can be as much as 30% if you manage to get everything exactly wrong. This part of the problem is dealt with by trying to keep the prop closer to parallel to the water at cruising trim. Eliminating the step might help some also. The way that works is it would shift the overall hydro drag forces aft. The relationship between the drag forces and the CG has a lot to do with these type of instabilities. Given the extreme weight shift resulting from you mods, it may not be correctable, but you might make it a bit less scary. I have seen a couple of these mods get sorted out and work okay.

You need to provide as much technical specs as possible. What speed and rpm does the problem develop? How far to you need to back off to make it go away? How deep in the throttle are you? What are the prop specs and engine gear ratio? What was the weight and cg of the stuff you removed? What was the weight and cg of the stuff you added?

 

The pod is described as owner built, if it is basically an extension of the hull bottom, but raised up 2", you may have intermittent contact with the water at the rear end of the pod, which would have a positive angle of attack, and being a leading edge, generate a good deal of lift. It may be cycling between water contacting the pod and lifting, then dropping down again. Most pods these days have a negative angle of attack, and are virtually not interacting with the water, once on plane. This porpoising issue might be aided by lifting the pod higher, but then you run into the problem of not getting full flow into the prop, having to drop the engine down, and getting more leg drag. The other alternative would be to align the pod with the bottom of the boat, as an extension of it. This would increase drag, but I'd not expect to have porpoising.

 

Yes the pod is owner built and it is very much like extension of the hull for these reasons I built the pod from marine alloy near full width 24 " long trying to neuralize the weight of the 225 because I can be travelling in 18 "" of water up and down gullies in between sand bars my first pod was a lot smaller and the boat sat stern heavy and would take longer to get onto plane sometimes struggle so went wider for more flotation and to reduce up to plane time New pod definitely improved on these made the motor weightless jumped up on plane in 1/3 of the time boat sit levelish were it was stern heavy Pod would start to porpoise at around 3/4 throttle to full would have to back right off to steady it when it porpoises it wasn't steady it would in crease every time the boat pitched down it would near jump right out of the water this was exaggerated by carrying weight at stern I. Use Yamaha prop 15 /15 Or 15/17 p. Even a 19p all depends on how much weight is in the towed tender 17 and 19p won't pull the tender on plane with 500 kg in it 15p will at around 44 mph 5600 Rpm not sure what you mean by cycling is this when the stern creates the lift then moves to edge of the pod stern to pod stern to pod so on
Think will do the easyist mod first motor wedge give an extra 5 degrees or so If that don't work I will lower the pod parallel to hull keel The extra drag wont bother me as it's not a race boat it's a work boat that tows its tender everywhere it does it very well pulls two boats up on plane just got some fine tuning issues wedge and reduce the step up
If I reduce the step should I put a negative angle or keep it parallel to keel line think I could only achieve 7 to 10 degrees negative angle