Nozzles and Fins

Our current project is a 50' Wharram. It has design displacement of 33,000 lbs
Power is two 150 hp Yamahas mounted amidships with 30" legs. I am wondering if Kort nozzles or ventilation plate wings will offer significant improvement in top speed.
Wharrams are doubled ended with a tendency to pitch. I am wondering if some horizontal fins aft or on the rudders would help this and als help the boat use some of this power.

 

I'm confused. Do you mean 150hp saildrives mounted midship in each hull, or a pair of 150hp outboards mounded aft near the centerline? No, nozzles won't/can't help here, they are just wasted surface area. That seems like about 10 times the power those boats need, btw.

 

That was exactly my thought. They only need about 30-40HP

 

I also do wonder were the 2 × 150 Hp (auxiliary ?) are for on a 50' Wharram sailing (?) Cat, but post #1 speaks about two Yamahas, so it looks to be outboards, and not saildrives.

One of David's previous builts was a 48' Ariki with 2 × 70 Hp Yamahas, for auxiliary propulsion . . .

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David shows one of the 2 × 70 Hp Yamaha outboards on the 48' Wharram Ariki at ± 11:31 in the above video.

 

WHOUW! What a horror Ariki! I always thought that Wharrams are simple, clean cruising boats and should be treated like that. Never thought that someone would ever put so much heavy stuff on a Catamaran. It must be sailing like a brick.

Cheers, Michel

 

I'll think BoatSmith's 48' Wharram Ariki is a nice boat for everyone who likes it that way, and an excellent built according to the client's SOR (Statement of Requirements), and it seems to attract new customers, hence the post #1 question. More input on that appreciated, I would say . .

 

Regarding the OP's question about the use of some sort of anti-squat plate to help with the top end. There was a pretty good thread about some traditional Mediterranean boats that have added aft sponson plates to their old displacement hulls and bumped the power up. I think it was in one of the propulsion forums, but I can't dig it up. The boats were Greek if I recall correctly. I suspect something like this could work here for modest gains in flatwater performance, but I have doubts they would do much crossing the Gulf Stream in a confused 6-8' sea.

 

Is this the meant thread Phil . . ? - - - - Displacement boat - speed​

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P.S. - Here's a related thread: Propeller size

Note this is the same boat as in the first above picture, but below with pumped up Hp and a sponson plate added.

 

Nice find. You obviously have a much better memory than me.

 

Actually it was you who remembered the threads, from your info I had a lucky shot with ‘‘sponson plate’’ on the ‘‘Search Forums’’ button at the top bar of this page.

 

About the anti ventilation plate wings in post #1, here's a related thread: Add On wings for Outboards

 

Are there real Kort nozzles available for outboards . . ?​

Or are you thinking about Kort nozzles special engineered and made for this purpose, meaning for the exact "boat/outboard/prop/location" that you are planning . . ?​

Note Kort makes a special fit between the prop and nozzle, and builds the nozzles with a special foil shaped cross section to make them work.​

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Kort Propulsion - ---> - Products - ---> - Kort Nozzles & Propellers​

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Splash Maritime - ---> - Books - ---> - Kort nozzles​

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I know there are Outboard Prop Guards with the same claims as Kort nozzles, but I wonder if those guards fit the prop well enough to work in such a way . . ?​

Also I don't see any foil shape in the cross section of the outboard prop guards that claim to work as a prop nozzle.​

Below an example . . .​

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All Points Marine Services - ---> - Inventory - ---> - ( Prop Guard Marine, Inc )
- 9 - XP White for 5 - 9 Hp Outboards
- 9" Yellow for 9.9 - 20 Hp Outboards
- 11" Orange for 25 - 35 Hp Outboards
- 13" Blue for 40 - 65 Hp Outboards
- 14" Red for 70 - 100 Hp Outboards
- 16" Black for 110 - 350 Hp Outboards & Stern Drives​

Their claims are listed in the right column of the below image . . .​

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. . . . . . How it Works . . . . . .​

. . . . . . . . . . . . ? ? ?​

The yellow Prop Guard means it's in the 9.9 - 20 Hp range, probably it's an exhaust through the prop hub, so I'm not sure what we actually see in the above picture ?​

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Those add-ons are usually called "hobbles" and can work well.

 

From Phil's ‘‘anti squat plate’’ and Gonzo's ‘‘hobbles’’ info, I've found this . . . .

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On the Wooden Boat Forums there's some talk about an Odd Skeg (aka: Anti Squat Boards, aka: Hobbles) on Derelict Boat

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Post #1: 12-29-2013 - Jerry W

‘‘ So, this old derelict boat was in the front yard of an antique shop on the Manteo, NC waterfront. Given that Manteo is on the Albermarle Sound which has very "thin water", is this skeg an outgrowth of those conditions ? ’’


↑ Dec 2013, Albemarle Sound at Manteo NC USA, Roanoke Island Maritime Museum, ± early 1900's ~ 1930's Sailboat Conversion to Engine Power, huge Anti Squat Boards, aka Hobbles, were added for this reason.

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Post #2: 12-29-2013 - Kevin Breakaway

‘‘ Those are known as squat boards and installed after boats were converted to power. Helped get more speed and/or a better running angle under power. ’’

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Post #3: 12-29-2013 - Oyster House

‘‘ Look at some of the round stern Chesapeake Deadrise workboats in the Kinsale area. Most will have some version a squat board. ’’

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Post #4: 01-02-2014 - Tom @ NC

‘‘ I live on Roanoke Island where Manteo is locate and know that boat well.

Those skegs are known locally as hobbles and were indeed an effort to prevent squatting that was a result of adding gasoline engines to hull forms designed for sail.

This was happening in the early 1900's through the 1930's or so, many boats were built for sail and as fortunes improved engines could be bought or bartered for. It must have been interesting for the folks involved to learn the dynamics without test tanks and computers. ’’

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Post #5: 01-02-2014 - Jerry W

‘‘ I can sorta look at a "hobble" and envision how it might keep a stern from squatting under power. ’’

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Post #6: 01-03-2014 - Ned L

‘‘ Yep, and also called 'squat boards' in N.J. where they were used on pound boats in the first half of the 1900's. They would help turn a "displacement hull" into a "semi-displacement hull" and allow it to get up on top & plane. ’’

 

I know that boat in Manteo. I used to live there.