IOM Do It Yourself DESIGN - Page 218 - RC Groups
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Jul 15, 2017, 06:38 AM
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There is scope to improve the shape of the aft deck to improve the airflow under the boom.
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Jul 16, 2017, 02:06 AM
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coolmobility's Avatar
Quote:
Originally Posted by RW1966
There is scope to improve the shape of the aft deck to improve the airflow under the boom.
Has whoever created these pressure maps, considered doing modelling with a raised foredeck (that closes the jib boom gap) to see the difference that standard change makes?

Looks interesting, but there are no yachts competitively racing with flat foredecks, that I know of. The comparison may even show that these raised foredecks are contrary, or an improvement, over a flat foredeck. Only by comparing two or more decks, can any assumptions be made. Still, an interesting and necessary baseline.
Jul 16, 2017, 07:53 AM
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Mast ram concideration as well
Jul 16, 2017, 07:09 PM
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The higher the mast ram the more effective it will be. RRII has the mast ram at the lower mast mark. A flat deck with a small elevated pylon to hold the mast ram may be a solution. One thing I have seen is some boats with semi flat decks to the bow act like spoons when the nose goes under down wind. The V9/V10 with the sharp water shedding nose works very well going down wind.
Last edited by davidjensen; Jul 16, 2017 at 10:50 PM.
Jul 16, 2017, 07:27 PM
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Quote:
Originally Posted by davidjensen
I'm not sure its radical but it is different. The curvature of the bow allows for a V shaped entry which I have not seen on a IOM to date. This design is wider than RR II especially in the bow area. The numbers are a little more balanced and I think this design may have an advantage going down wind in light to moderate air, but it has to be built to find out. The amount of rocker in the bow may cause some helm issues in heavy weather trying to tack but other than that it is a departure from the norm. I guess you can say its radical.
If I might presume to offer a comment, it would be that your bow as designed will sacrifice LWL and hence potential speed at just about any angle of heel. It could be argued that the percentage loss would be insignificant, but it's there and it seems to me that designing a better mouse trap is a matter of seeking out fractional gains, not the reverse.

I have it on reliable authority that when the Britpop -- which most IOM people would consider a breakthrough design -- first came on the scene some scientifical-minded people in Sydney did some empirical testing. They found the BP was one per cent better than known designs. One per cent represents 1m in 100 or a boat length. I suggest the moral is obvious: gather ye rosebuds where ye may.
Jul 16, 2017, 09:31 PM
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The new Kantun 2 also contradicts this idea of LWL rather well. I saw Zvonko sailing the prototype in San Diego and the nose was well out of the water quite often. I'm no expert but the V9 and V10 are also "breakthrough" designs IMHO and I'll bet they are about .25% better when going down wind with their noses buried.
Last edited by davidjensen; Jul 16, 2017 at 10:51 PM.
Jul 17, 2017, 06:55 PM
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Quote:
Originally Posted by davidjensen
The new Kantun 2 also contradicts this idea of LWL rather well. I saw Zvonko sailing the prototype in San Diego and the nose was well out of the water quite often. I'm no expert but the V9 and V10 are also "breakthrough" designs IMHO and I'll bet they are about .25% better when going down wind with their noses buried.
The fact still remains that displacement hull speed is limited to 1.3 or 1.4 times the square root of LWL i.e. at sub-planing speeds, the longer the waterline the faster the potential top speed, disregarding other factors such as wave action.

As to the advantages of fine bows may I again offer this clip of the Frigate Mark 2

FM2 overpressed downwind (2 min 11 sec)


Note that despite the bow burying as far back as the mast several times, the boat does not trip. The other ingredient in maintaining control is the oversize RM rudder which has sufficient depth to stay submerged. The actual boat looks like this:
Jul 18, 2017, 03:11 PM
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FWAL's Avatar
Quote:
Originally Posted by coolmobility
Has whoever created these pressure maps, considered doing modelling with a raised foredeck (that closes the jib boom gap) to see the difference that standard change makes?

Looks interesting, but there are no yachts competitively racing with flat foredecks, that I know of. The comparison may even show that these raised foredecks are contrary, or an improvement, over a flat foredeck. Only by comparing two or more decks, can any assumptions be made. Still, an interesting and necessary baseline.
Ledge had a flat foredeck. She made the 'A' fleet in a couple of UK Ranking flights so wasn't a complete slouch. The skipper was the slowest element of the design!
Jul 19, 2017, 03:16 AM
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Quote:
Originally Posted by FWAL
Ledge had a flat foredeck. She made the 'A' fleet in a couple of UK Ranking flights so wasn't a complete slouch. The skipper was the slowest element of the design!
Hi FWAL,
I was referring to the totally flat deck of continuous flat deck from back right through to front, as is shown in the airflow diagram. Your foredeck top, although flat on top, is raised and very close to the jib boom, which would radically change the airflow across the deck. That is the aspect I was referring to.

I love your designs, and have even incorporated my sidestays hooking into the deck, as you explained to me in PMs, and after discussing this feature with you, I've done similar by tubes going forward, instead of into hull, under deck, like you do. My jib sheet is also coming from under the foredeck rear, like you do.
Jul 19, 2017, 09:52 AM
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I would love to see a side view of the Frigate and the water line. With that narrow of a bow section she must have a lot of rocker in the bow. Does this create any helm problems in high winds?
Jul 19, 2017, 08:19 PM
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Quote:
Originally Posted by davidjensen
I would love to see a side view of the Frigate and the water line. With that narrow of a bow section she must have a lot of rocker in the bow. Does this create any helm problems in high winds?
Herewith. Not really anything to see here.

Critics (friendly) kept saying that it would never be able to tack with the high bow in breeze. Not a problem.

Some years ago, I did try a design based on an Australian surf boat, with a lot of bow rocker. Not good. No problem tacking as I recall, but generally slow except in very specific conditions when it would surf like a demon and leave everything else for dead.


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