Porpoising

I know it is my comment, but you said it is not true while I still think it is. Excessive trim for the boat in question at the speed in question. There is no single value of trim limit for a given deadrise and boat. It will vary with speed. The faster you go the lower the trim limit will be. Making the hull narrower (or heavier) enables to use higher trim at the same speed.

There is a lot of measured data about porpoising done by different people in towing tank and at sea for the last about 100 years. It was a big safety issue with flying boats.

I think it is well shown that it is the trim, which makes the porpoising start, not how the trim is achieved. Naturally the thrust line or tow line will affect the trim, but not porpoising at the same trim. So you may need different CoG to get the real trim, but that doesn not alter drag nor porpoising.

 

Fitting trim tabs (to the hull proper, not the pod) would be a useful test, if he actually still has enough width left either side of the big pod. For boats with pronounced porpoising problems, appropriately sized tabs are far more effective than engine trim in getting rid of it. I still think that pod is a dud though.

 

As most of our boats were built with a 12 degree hull, with a 7 foot chine width, I will use these as examples though I can offer other situations where porpoising started with different deadrises, loading and speed. Almost all were jet drives though some were IO and outboards

Our hulls with a normal 330 hp and a top speed of around 50 mph did not porpoise. Except once. The boat was loaded with a standard load but on this on occasion, we had stowed an additional 150 pounds of fuel in jerry cans at the back of the boat. The water was absolute calm and when we reached about 40 mph, the trim angle of our boats ran close to 5 degrees up at this speed, the boat began porpoising. 5 degrees is not extreme or excessive. We held this speed and within about 15 seconds the hull was porpoising with increasing amplitude and was becoming uncontrollable. We throttle back to 35 prox and the porpoising quit. We took the 150 pounds of fuel and moved it just past midship and increased the speed back to 40 mph and there was not a hint of porpoising. This slight change of the center of gravity, cured the problem.

As we built out of aluminum all of our hulls from 18 to 24 feet came off the same jig. One customer wanted an 18 foot boat capable of 65 mph so he provided a fire breathing V8 and we built the hull the standard way.

On test day we ran the boat up to about 50 mph and the porpoising was so bad we had to shut it down and rethink our position. The solution for us was to install a Jetovator which we could manually change the thrust direction from the helm. We just trimmed the bow down a couple of degrees and never had another issue. Based on this experience, it is my opinion that it excessive trim angle is not the culprit but rather the relationship between the various dynamic drag on the hull, the thrust line, the center of gravity and the center of lift that creates the porpoising problem.


Those of us who have an overpowered tender have probably run into this same situation, get over hump, have the bow trim down to a normal trim attitude, and the slow effect begins.

We had been called in to deal with a serious pitching problem on another instance that shows what can happen when the center of lift gets behind the center of gravity.
The hull, not one of ours, was a custom aluminum hull, inflatable tubed 14 foot boat that had an outboard jet and a tunnel in the aft 1/4 of the hull to permit the jets intake/shoe to be above the keel line of the boat
Additionally, the manufacturer, had installed a bow mounted tank of approximately 50 gallons. Say 400 pounds

The boat would accelerate to around 30 mph, with a trim angle of around 8, deadrise around 25 degrees prox, and then the bow would drive violently down and to the right or left. The effect was impressive.

Because the fuel tank was in the front with considerable weight, it produced a very front forward center of gravity.
As speed increased and the center of lift moved backwards, the boat lost the ability to keep the nose up. What we felt aggravated this extreme change in attitude was the fact that the tunnel area did not provide any lift so when the center of lift got close to the tunnel, the lift just collapsed

The solution that we used to fix this was to shorten the tunnel and move the fuel tank to the back. The owner could then trim out any tendency to porpoise depending on the rest of the load.

 

OP has an Regal Empress, which seems to be a 20' boat. LOA 5.95 m, beam 2.44 m and displacement 1600 kg. I'm guessing deadrise is 15 deg and using 2000 kg displacement with some fuel and crew.

At 15 knots the porpoising limit trim (keel line to water at speed) is 7.0 degrees, which equals to CoG 2.05 m from transom. At 20 knots the limit is 4.6 degrees and CoG 2.25 m. At 30 knots the limit is 3.7 deg and CoG 2.05 m. At 40 knots 3.5 deg and 1.75 m. At 50 knots (requires more than 225 hv) 3.4 deg and 1.5 m. The CoG and its change with speed will depend on thrust line.

If it had 30 deg deadrise, the valus would be: 15 knots 8.0 deg 2.25 m, 20 knots 6.8 deg 2.35 m, 30 knots 5.9 deg 2.2 m, 40 knots 5.6 deg 1.9 m and 50 knots 5.4 deg 1.65 m.

If a 450 kg stern drive + 4.3L was removed with CoG say 0.5 m forward of transom and then 350 kg pod + Honda 225 (270 kg) was added with CoG say 0.5 m rearward of transom, the CoG of the boat was moved about 0.1 m rearwards and got 100 kg lighter. If the pod is heavier so that the added weight equals to the removed one the CoG moved 0.23 m rearwards.

How was the old engine space used? Was the fuel tank moved there etc.?

By excessive trim I meant more than the limit of porpoising. So in the case above at 40 knots 4 degrees would already be excessive.

 

Those are the things that determine the trim of the boat at speed so we seem to agree that trim angle is the key. To stop porpoising you need to do something to lower the trim like moving weight forward or lowering the trim of the OB/stern drive or deflecting the jet thurst downwards.

 

I'd imagined he was talking of a somewhat bigger boat, 24 or 25 feet. A 225 is overkill (and overweight) unless it is a deep vee.

 

I agree with this solution, especially if overpowered.

 

Hi guys some very good reading and info coming my way and have pretty much worked out what went wrong it doesn't seem to be the pod scrap that a hull extension is more like it that is the problem as the boat doesn't porpoise all the time most of the time could trim down and settle the boat at speed I wend and picked up my mooring chains about 250 kg and had my son come out with me maybe 90kg. all that weight was at the stern so. The COG. Was moved to he stern a lot what made it porpoise to unstable my dud pod gets me up on plane twice as fast and having such large pod with its displacement helps lift that big motor and the boat sits nice in the water not arse deep as with previous pod use such a over kill motor because I'm dragging out the water getting on plane probably about 3 ton in all So going to make some tweaks to pod see if I can improve it first going to fit a transom wedge give me more angle when I'm carrying to much weight Also what do ya all think about this a high lift plate fitted to the keel ov the pod as it is straight parallel the the boat I could add a negative angle to the pod only about two inches over 24" length mite give me 5 to 6 degrees negative thinking ov making high lift plate 8" wide could easily be added There's no room to fit trim tabs to the boat but could be fitted to the pod The obvious solution is to remove all the weight from the stern so COG moves forward A transom wedge would help What do think about a high lift plate 24" long 8" wide 6degrees up negative Surly these would help All so when I designed my pod I left about 30mm at the bottom along the entire width of pod set the transom board back So I could bend the edge down a bit a bit like full width trim tab I'm not a boat designer or racer So it's all bit hit and miss very grateful for all your inputs helped work out what was wrong COG.

 

Hi guys from across the pond and elsewhere thankyou very much for all your input On poiposing some very interested reading Woi oi. I have fixed the problem to negative angle pod runs sweet as a dream now the pod was not at fault I would never achieved what I needed if I made positive angle pod would have my my boat sit rear end heavy So my pod was near full width and folllowed all the angles of the boat extended by 24" neutralised the weight of 225 honda gets on plane in seconds towing a 1 ton boat dosnt dig in Anyhow how just wanted to let you all know that problem is fixed by removing transom plate and removing 4 degrees making a wedge also removing excess weight at the stern. Runs true not a bobble and can trim up to the sweet spot All good So keep it simple and thanks again for giving me so much info so I could work out what the actual problems were. Cheers from England Ev1

 

Congratulations mate. Good job.

 

Too much like hard work reading that explanation, I'm afraid !