Planing Speed

[senator]

The other week we were coming North to South on Breydon behind a very nice Blue boat possibly a Bénéteau,

We were at our lowest planing speed which on the day was around 22 knots, the other boat which looked a lot heavier than our 3 tons was also planing but at a much lower speed possibly as low as 15 Knots. How is this possible?

The other skipper also owes us a bottle of wine as when he got up to the turn for the Yare he turned round and accelerated onto the plane, he must have gone faster though because I would say he was coming at us at at least 50 knots and causing massive possibly as high as 6' waves to roll across Breydon, in order to counter these tidal waves I had to thrust forward the throttles causing my bow to fly out of the water and the bottle of wine which obviously was in the right place for Breydon, open and blanced on the edge of the galley work top, to be thrown off onto the floor consequantly smashing and ruining our whole years holiday.

Seriously interested in how kingfisher gets on the plane so early and just thought you might be missing the second bit if you had'nt been on NBF for a while

Ian

[Brian]

I have Bennet Trim Tabs on the back of my Bayliner, this provides extra lift at the back to get it on the plane sooner, ive also found it planes a bit slower.

[senator]

Brian,

Have trim tabs and the boat and it used to stay on the plane down to about 17 or 18 knots but with the addition of all the cruising and convienience weight she is now a fair bit heavier, was thinking of fitting Doelfins to the outdrives if enough time is spent at planing speeds to justify it, does anyone have any expierience of these, do they work?

Ian

[pks1702]

x

[pks1702]

x

[senator]

That is exactly the one Perry, did you get the reg number so I can report him to the cheap spanish plonk authority.

White lady does start to climb at about 12 knots but is heavily on the hump right up to about 19 and it is only then that she will climb over and levels off at about about 22 knots with the rev counters both showing around 3500, knock it back to 3400 and she just sinks back down.

Fastest I have had is 38 knots on Breydon at high tide, this was supprisingly the same by both paddle wheel and GPS, and even more supprising was the fact that on sea trials down in Poole she wouldnt do any more than 32 and that was with no weight on.

Is it possible that distribution of weight could alter the performance characteristics so much?

[Antares_9]

Not me Ian, probably Rachel at the helm as I'm frightened to go fast

Whte Lady is a deep V or a modified V which gives less lift for each kt of forward motion than the type of hulls Perry and I have, they are thick at one end, much, much thicker in the middle and thin again at the other end, no wait, that's Dinasaurs.

They are (unlike Dinasours) quite pronounced V at the bows then quickly flaring to much flatter sections aft so a bit like a tea tray that gives more lift for a given speed, it also requires a bit more power to reach that given speed. Also as Perry has said they are able to behave better at river speeds, especially if they have a bit of a keel. However, as in all things there is compromise, if I were to be able to shoehorn enough power to reach White Lady's speeds then in aything other than a flat calm she would slam, crash and probably break the hull (oh no not again) A deep V entry, leading to say a medium V at the stern will give a much softer ride in a bit of a chop at higher speed. Yer pays yer money, yer takes yer choice.

Trim tabs only make a little bow down difference and I really can't see why they were fitted to the Serie 9, on a deep V however they are essential, not really to assist over the hump, drive trim is better at that, but rather to help reduce list in a side wind or beam sea.

I'm really sory about your wine and I will have my man "bring round" a case to replace it when I am able

Edit to answer your last question Ian, yes, loads, the more stuff and peeps at the back, the worse it becomes.

[senator]

Have moved batterys from the stren on the port side to more midships and all the food microwaves and clothes etc. is forward, six knots extra though with more weight on board seems a lot. Having said that it was blowing 5-6 and we were pushing over 2-3' waves on the sea trial.

No one had any dealings with these doelfin things then? have heard they are really good on small ski boats and the like but not heard anything about them on a 27' twin engine set up.

Ian

[pks1702]

x

[senator]

White wine perry, but it has taken all the colour out of the red carpet

[Antares_9]

Have used a "Doel Fin" on an underpowered outboard boat and it did lift the stern a bit more, also made the boat a lot more "skittish" at speed. If you ever, EVER consider that you may find yourself in any kind of following sea then they are a really bad idea and produce even worse handling (just the dangerous side of scarey) than tabs down in a following sea. and you may find your stern trying to overtake your bows more than is usual in those conditions.

[senator]

Will give those a miss then thank you David, dont fancy planning to go against the tide all the time..

Ian

[Guest]

Only just caught up with this.

Silver Dream has a deep vee entry which turns into a 3/4 length keel and almost fully round aft section. The keel is quite pronounced. She climbs out at around 13kts and can sit at that speed all day and all night. In fact, to be honest, she's virtually planing at around 10-11kts but you have to keep the tabs more or less fully down to keep her there which obviously causes drag and is not the most fuel efficient way of running.

There is some degree of argument as to whether the Mirage is a planing or semi-displacement hull. Now David will tell you there is no such thing as a semi-displacement hull so on that basis it is a planing hull but there's no doubt it is quite a different design from the typical sports cruiser. I also don't experience any of the handling oddities at low speeds which I read about from others - that and I can stay up at much lower speeds.

Of course SD is not a typical Mirage anyway. Generally they were powered with twin 145 petrols and rated with at 26-29kts flat out. I have my doubts that Mirages with that motive power could ever have actually achieved those speeds, I suspect it was Fairline doing a marketing job on it. An acquaintance of mine, Martin, owns a Mirage called Artisan (pictured in my avatar) which was the original show boat for the MkII Mirage (then called Aine) and powered by updated 145's. She now runs on two marinised Ford diesels which also put out 145hp each and will do about 26kts full tilt but spinning bigger props than the petrols could have dreamed of.

SD is powered by two old Penta diesels from the MkI Mirage era. At nearly half a ton each they put a massive amount of weight slap bang against the transom and need to be on top form to run well. I've had her up to 16-17kts when running on Breydon with Kingfisher, she sits at around 15kts if I throttle back to 3500rpm (4000rpm is the max). At that speed the engines sound like a light aircraft (!). I know someone else with an almost identical Mirage - she's a little older and has some small differences but essentially the same. He won't take his over 12kts now because the engines are worn out (they've done over 12,000hrs, mine have done 1600hrs) but in her heyday when he had her on the Solent he used to be able to hit 20kts with low fuel and water - 18-19kts was more typical when more heavily loaded.

I doubt our old girl has seen serious action for years because she came from the canals. In fact as far back as I have any information she's never seen any serious use which would explain the ridiculously low hours on 30 year old engines. With Uncle Gordon putting the cost of fuel up I can't see we'll ever do any passage making now so those old lumps will have to get used to trundling along the Yare at 1200rpm. They never really were the ideal pairing for a Mirage design anyway; two big and heavy, not really powerful enough, very close together and they do rattle a bit at river speeds but Neil tells me it would be like £16k each to get any up to date Volvo's so I guess we might as well get used to them!