I would say that is it not specific to proas. Other multis use certain hull forms to counter the bow down trim. Based on Rick's messages in earlier discussions the same can be done with proas too but with a bit different approach due to the fore-aft symmetry.
BR
Arto
From: bjarthur123 <bjarthur123@yahoo.com>
To: harryproa@yahoogroups.com.au
Sent: Sunday, July 17, 2011 3:38 AM
Subject: [harryproa] Re: bow down trim
thanks.
so the short answer is yes, the bow down problem is specific to proas due to their fore-aft symmetry.
do i recall correctly that your graphs of trim vs. speed were made by adjusting only the coefficient of lift of the sail at speed or maybe its AOA, and specifically not the height of the lift? in other words, maybe the trim wouldn't be so bad if the sail was reefed?? not possible with your snazzy wing i know, but applicable to most proas.
your four ideas on hull shapes all make sense to me and are very good in that they require no moving parts. i still can't help but wonder though whether the best solution is a pantographing ww hull. it would put the weight right where you want it. such freedom of movement could even possibly be useful as a folding mechanism when trailering.
ben
(RC'ing for J/24's tomorrow,
next weekend crewing on a corsair 28cc,
i miss my weta already!)
> The main difference between say a catamaran and a proa is that the
> former can be set with an initial trim using weight distribution.
>
> Also the transom on a typical cat hull will suck the stern down at
> speed. This increases bow up trim and the angle of attack for the
> hull so there is increased dynamic lift compared with a hull that
> rides flat.
>
> With a clean canoe stern there is not as much sinkage in the stern.
> This means there is greater tendency to ride level under power. If
> the drive is high up then there is a large bow down moment that will
> force the bow down unless the hull can counter it.
>
> So far I have observed four ways to get the hull to provide bow up
> moment.
> A. At displacement speed having full ends (large waterplane at the
> bows) will increase the static pressure from the bow wave at the
> leading end and reduce the pressure at the trailing end. This can
> give bow up moment below planing speed. For high speed displacement
> hulls the full ends also produce the lowest drag hull.
> B. Flaring the sides of the hull increases the waterplane area in the
> front end and reduces the waterplane area in accordance with the hull
> waves noted in A to increase the moment created by the hull waves.
> C. The longer the hull the greater the moment from the lift and
> sinkage created by the entry and exit.
> D. Having a flat planing surface in the ends with a slight rocker
> will provide dynamic lift once the hull approaches planing speed.
> The rocker on the trailing edge will create suction so it will assist
> with the bow up trim.
>
> It is certainly a key issue for consideration with a large proa where
> it is not convenient to move ballast around after each shunt
> although plenty of racing keel boats use their crew as movable
> ballast. However I do not believe it results in a limit in speed if
> the hulls are designed to stay bow up under the drive.
To: harryproa@yahoogroups.com.au
Sent: Sunday, July 17, 2011 3:38 AM
Subject: [harryproa] Re: bow down trim
thanks.
so the short answer is yes, the bow down problem is specific to proas due to their fore-aft symmetry.
do i recall correctly that your graphs of trim vs. speed were made by adjusting only the coefficient of lift of the sail at speed or maybe its AOA, and specifically not the height of the lift? in other words, maybe the trim wouldn't be so bad if the sail was reefed?? not possible with your snazzy wing i know, but applicable to most proas.
your four ideas on hull shapes all make sense to me and are very good in that they require no moving parts. i still can't help but wonder though whether the best solution is a pantographing ww hull. it would put the weight right where you want it. such freedom of movement could even possibly be useful as a folding mechanism when trailering.
ben
(RC'ing for J/24's tomorrow,
next weekend crewing on a corsair 28cc,
i miss my weta already!)
> The main difference between say a catamaran and a proa is that the
> former can be set with an initial trim using weight distribution.
>
> Also the transom on a typical cat hull will suck the stern down at
> speed. This increases bow up trim and the angle of attack for the
> hull so there is increased dynamic lift compared with a hull that
> rides flat.
>
> With a clean canoe stern there is not as much sinkage in the stern.
> This means there is greater tendency to ride level under power. If
> the drive is high up then there is a large bow down moment that will
> force the bow down unless the hull can counter it.
>
> So far I have observed four ways to get the hull to provide bow up
> moment.
> A. At displacement speed having full ends (large waterplane at the
> bows) will increase the static pressure from the bow wave at the
> leading end and reduce the pressure at the trailing end. This can
> give bow up moment below planing speed. For high speed displacement
> hulls the full ends also produce the lowest drag hull.
> B. Flaring the sides of the hull increases the waterplane area in the
> front end and reduces the waterplane area in accordance with the hull
> waves noted in A to increase the moment created by the hull waves.
> C. The longer the hull the greater the moment from the lift and
> sinkage created by the entry and exit.
> D. Having a flat planing surface in the ends with a slight rocker
> will provide dynamic lift once the hull approaches planing speed.
> The rocker on the trailing edge will create suction so it will assist
> with the bow up trim.
>
> It is certainly a key issue for consideration with a large proa where
> it is not convenient to move ballast around after each shunt
> although plenty of racing keel boats use their crew as movable
> ballast. However I do not believe it results in a limit in speed if
> the hulls are designed to stay bow up under the drive.
__._,_.___
.