Moving Rudder Position

[GRIT]

Originally Posted by Fore and Aft

CMNZ I see your fitting hydraulic steering so you are not going to get any helm feed back compared to a cable system. So I would think that any extra load on the rudders is not going to be noticed by you if you move them aft.
Grit I was referring to the other thread "Don't believe everything you read"
Cheers

AH, I get it. Thanks for the clarification.

Cheers.
Paul.

[Seaslug Caravan]

Originally Posted by CMNZ

I am completing the building of a Simpson 11m which has been lengthened to 11.6m by adding sugar scoops.

Below is the original plan showing the original waterline and I have drawn on where the new waterline will be (approximately) when the rudder is moved aft.

CMNZ, firstly congratulations on your build. Fewer and fewer big boats are being home built these days.

Remember the Simpson 11 is a very light vessel by today’s standards. Less than 4.5 tons from memory. Designed before people put targa bars covered in solar panels and the great big motorised heavy dinghy on Davits down the stern etc etc.
Adding extensions and moving your rudders aft is only going to sink the sterns even more. From memory again, Rogers rudders on this size vessel we’re using at least 50 mm solid SS bar. They are not light.
Your guess at the new waterline is very optimistic.

Don’t forget the emergency steering that Roger designed for very good reason.
If you decide to flick the rudder skegs then the existing stocks will be too flexible.
Posting these questions on a public forum is probably not the best, pay a NA and get them to do a weight study and tender their professional advice.

[Fore and Aft]

Seaslug you could do away with the stainless steel bar and just use two hydraulic rams.
Cheers

[CMNZ]

Originally Posted by Seaslug Caravan

CMNZ, firstly congratulations on your build. Fewer and fewer big boats are being home built these days.

Remember the Simpson 11 is a very light vessel by today’s standards. Less than 4.5 tons from memory. Designed before people put targa bars covered in solar panels and the great big motorised heavy dinghy on Davits down the stern etc etc.
Adding extensions and moving your rudders aft is only going to sink the sterns even more. From memory again, Rogers rudders on this size vessel we’re using at least 50 mm solid SS bar. They are not light.
Your guess at the new waterline is very optimistic.

Don’t forget the emergency steering that Roger designed for very good reason.
If you decide to flick the rudder skegs then the existing stocks will be too flexible.
Posting these questions on a public forum is probably not the best, pay a NA and get them to do a weight study and tender their professional advice.

Thank you for your comments. I believe the boat is likely to end up around 4.5-5 tons. It was almost exactly 3 tonnes when craned into my property at hull and decks stage I am also going to be using outboards so there is a huge weight saving in the stern right there. The design calls for 40mm SS stocks and I will shape them from ply so they will likely be almost buoyancy neutral. I am not planning on removing the skeg. The sugar scoops weigh very little.

The waterline I drew was relative to the rudders rather than actual. The original waterline will remain the same but the rudders rise up about 130mm when moving back 600mm so I will extend them by the same amount and slightly reduce width to maintain a similair surface area under water.

Thanks
Craig

[Seaslug Caravan]

Originally Posted by CMNZ

Thank you for your comments. I believe the boat is likely to end up around 4.5-5 tons. It was almost exactly 3 tonnes when craned into my property at hull and decks stage I am also going to be using outboards so there is a huge weight saving in the stern right there. The design calls for 40mm SS stocks and I will shape them from ply so they will likely be almost buoyancy neutral. I am not planning on removing the skeg. The sugar scoops weigh very little.

The waterline I drew was relative to the rudders rather than actual. The original waterline will remain the same but the rudders rise up about 130mm when moving back 600mm so I will extend them by the same amount and slightly reduce width to maintain a similair surface area under water.

Thanks
Craig

Excellent, sounds like you have it sorted.
Out of interest, can you tell me the drawing/design date shown on the plans.
I have always admired simpsons eye for a pretty boat and would love to know where the 11 fits into his design catalogue.

[CMNZ]

Interestingly only one of all the plans has a date on it, Oct '89.

Here is a link to Simpsons plans list:
https://boatcraft.com.au/Shop/index....ex&cPath=32_33

[Cheechako]

I doubt it will matter much at all.
First; the few inches aft wont effect the CLR enough really. I've tried to adjust the helm feel on two boats over the years and was very surprised how little it effects.
One 30 ft boat I added a 3 ft bowsprit for the headsail. It's helped a little but it sure wasn't huge. Probably call it 2.5 feet as the fitting was back from the tip a bit.

The other was more like a 2 ft extension to the headstay. Neither solved the weather helm issues.

Think about changing your 140% genny for your 100%....
Remember, your helm changes every time you change a headsail or reef.... there's a lot of fudging to the entire thing in the first place.

On the first boat mentioned above, I later put on a modern big bard door rudder in place of the rounded old school "1/4 moon" style rudder. That added a lot of wetted area aft. I really couldn't tell any difference.

[Pauls]

One issue with the rudder extending above the waterline is that it will ventilate when the helm is put over a fair amount. Ventilation means the suction side of the rudder, instead of pulling in the water flow against itself, instead sucks down air. This reduces rudder power noticeably.

[Redreuben]

Add your 600ml by spacing out the frames aft of the middle of the boat, not by tacking on a bit at the back.
That will keep waterlines and rudder depth unchanged.

[Pauls]

Regards Redreuben's suggestion, YES!

[chrisr]

most posters seem to have overlooked the original question which related to the small section of the upper rudder out of the water

firstly, i expect that while sailing you will find this tiny bit of rudder is going to be immersed anyway (albeit in turbulent ie not effective water) due to squat and stern wave

secondly, you mention you are lengthening the rudders, so looks like you will gain total area after all

frankly, i wouldn't worry about it. doubt if you will even notice

fyi we once went 2 or 3 days before noticing that the hydraulic steering had become completely detached from one rudder (admittedly the weather was kind...)

as regards the simpson, an earlier cat we had was a simpson cloud 9, with transom hung rudders. we fitted hydraulic steering to this one as well...single double acting ram mounted midships, with adjustable rods to each tiller. worked a treat !

cheers,
.

[catsketcher]

There will be absolutely no problem with moving a rudder aft. Moving a rudder aft increases its "volume" - its area times its lever arm. That is almost always a good thing. If the rudders are loaded (weather helm) then they will take slightly less side load but will do so more effectively. If you see skiffs like 12s, 14s, 16s and 18s, they all have moved their rudders aft on gantrys - it improves rudder response. The boat will likely be a little bit more manouvrable because of the change.

As has been stated above, as soon as the boat moves it will make a hull wave, then the rudders will be fully immersed. We can easily see this monohulls that often show their rudders (especially old IOR types) that then had well immersed rudders underway. This will not be a problem. Although people may say they don't, cats make hull waves. Your boat will make a fair wave at 6 knots, you may not see it in a chop, but on flat water, that hull wave will immerse the rudders fine.

As for the rudders and the hull - keep them tight to the hull. This reduces a phenomena called tip vortex. A foil with an endplate (like an underhull rudder) has an effective aspect ratio of about double that of a non endplate rudder. Racing rudders are usually underhung for this reason, surface piercing ones can ventilate, plus they don't get the tip vortex and aspect ratio advantages.

There is a lot of bumkum about rudders - I did a fair bit of experimenting when making my folding cats. I finally found out that making big rudders made the boats really well behaved. I think most cats have rudders that are too small, which is why many cats tack poorly whereas slow monos tack so well. Monos have lots of underwater area compared to most cats and this area is under low loading at slow speeds and does not stall so easily. Of course most cats have rudders that are limited in size by depth limits of the minikeels but you can get around that by using kick up cassettes.

Cat rudders get very highly loaded as the boat comes out of a tack, the mainsail roach fills and with low speed the smallish rudders stall. Make rudders big - but unless you use cassettes and kick them up you will run into problems when beaching. You could pay a designer or naval architect, or you could just find a similar sized cat, with a well proven rudder design and use the guts of that rudder (shaft size and area is the main thing). My little cats use 16ft skiff centreboards as rudders - they work like a charm. (My 38ft cat of similar weight uses 50mm shafts with 6mm wall stainless tube - worked well for 22 years)

My little cats can tack under main only, and can even bear away from a stall with no jib up, by gently pulling on the main and giving a little pump on the tiller. They are so forgiving. But pull one rudder up and halve the area - and they turn into typical slightly cantankerous cats. My contention is that most cats have too small rudder area for nice manouvring.

But have you built the extension yet? - if not then stop and start a thread on that. Many extensions are just continuations of the waterlines - you should not do that if you have a partially built hull - there is a better way that will increase aft volume and you should think carefully about an aft deck and steps. The whole extension should be integrated.

cheers

Phil

[Chotu]

This is another vote for it not mattering at all. Your rudder can definitely be above the waterline by the amount you show. Mine is and it turns like a well designed race boat. Extremely responsive to the slightest helm change.

As to lengthening the boat, spacing the stations out is a better way to do it. I wish I had thought of that myself when I started mine. I added reverse bows and a little bit to the aft end.

You definitely need to keep the rudders as nestled up to the hull as possible. It’s important to minimize the gap there so you don’t lose performance.

[CMNZ]

Originally Posted by Redreuben

Add your 600ml by spacing out the frames aft of the middle of the boat, not by tacking on a bit at the back.
That will keep waterlines and rudder depth unchanged.

Way too late for that, I am afraid, see:

https://www.youtube.com/watch?v=sIMUQGo6X_U&t=17s

[Redreuben]

Oh ! Lol ok.
In that case I agree with what has been said, difference would be minimal.
You’ll just have to sail the boat hard pressed in all conditions to keep the rudder wet.
Disclaimer;That was a joke. 🤣🤣