13' skiff design

Hello All,

New member here. I am currently in the early stages of designing a light weight skiff for use as a tender to my sailboat.

My goal is a durable soft riding light skiff that can carry four people and a load.
I am a year away from a world cruise with my family. So this is going to be our main transport to shore.

The reason I am not going with a 11'-13' rib is the tubes are not durable long term (longer than 10yrs) and the ride is not soft enough for my bad back.

The skiff is 13' x 6'
Dead rise at the transom is 24 degrees
Centerline adjustable water ballast
construction is going to be carbon skins, foam core, epoxy infusion, and Kevlar rub strips on the chines of the hull.
My friend has made a 11' rib with same construction, but different design and the hull weighted 65lbs

The estimated weight for the hull is 125lbs (no water ballast)
engine 115lbs
fuel 50L 85lbs
misc 25lbs

total 350lbs.

A question I have is regarding the displacement of the skiff when at rest.
I have a water line area of 65 sq.ft.
I am concerned that the boat will sit too high in light displacement and rock on the vee when at rest.

I have been using this equation as a guide:
lbs/in. = WPA * 64/12

thus 350lbs/in. =65.6sq.ft. * 64/12

I understand that the equation assumes a flat shape and the vee would change immersion, but how much.

Any comments are welcome.

Thanks

Gavin

 

Welcome to the forum. Surely here you will find solution for your skiff.

A lot, to say a number (but would have to estimate the volume of buoyancy with those forms) at least 2/3 the height of the "V".

 

Would it be correct to find the volume of the vee and then multiply by 64lbs to find how much weight it would take to submerse the vee?

 

You have to calculate the volume of the hull to various drafts, that value multiplied by the specific gravity of water, will give you the weight (skiff + load) can support the skiff to each draft.
In any case, the "V" seems too steep for such boat. Think about 6 or 7 degrees.

 

This is impractical, you will not create a 24* bottom 13' boat, especially a lightweight one, that is not hopelessly tippy. TANSL is right, you will have to go much flatter bottomed. The only boat I know of that vaguely fits the description, weighs well over 500 lbs, which is a hell of a lot for a 13 foot open boat. Google "Cootacraft Little Ram", they have youtube videos as well. That is a retail boat, not plans.

 

What if I raise the vee relative to the sides to reduce the volume of the vee and increase flat area. I am willing to give up efficiency to gain a soft ride.
Will the hull become stable if the corner chines are immersed? What angle stern to bow do people think the hull would sit at?

The rib that my friend built had a 45 dead rise with flat area in the transom and tubes for stability. There is a rib out of Europe that he copied.

 

Google "Cootacraft Little Ram", they have youtube videos as well. That is a retail boat, not plans.

Thanks for the Little ram link.

 

Welcome to the forum.

Why such extreme deadrise? Yes, it'll be quite tippy static and at displacement speeds. She'll also tend to be rolly on plane, unless you're using a lot of power (50 HP). Your engine weight suggest a 15 HP outboard, which will likely find getting this puppy up on plane with "4 people and a load" quite difficult. Lastly, your questions suggest you don't currently have the expertise, to design as specific a set of requirements into this boat. There are lots of 12' - 14' powerboat designs to choose from, what about these isn't suitable?

 

Bad backs and lightweight, small fast boats is not a compatible mix, you have to compromise, and slowing right down, and optimizing for a stable platform, seems the best answer to me.

 

Agreed, high vertical acceleration loads and a high speed hull, even with a ridiculous amount of deadrise, for a boat of this type/size will mean lots of impact tolerance is required. Consider semi displacement speeds, with a built down hull form of some heft, for comfort and much higher fuel efficiency too.

 

Another well meaning person who has deluded himself into thinking that he/she can design a boat to satisfy a specific purpose. It looks easy but it is actually one of the devils schemes to torture the unaware.

 

Yes this is a learning process for me. I am here to ask for help and suggestions. The attributes that are different in this project are quality materials, water ballast, not for resale.

Let me requalify my needs:
Light weight as I plan to use this dinghy from a performance catamaran hence the quest for a light dinghy when I lift it out of the water.

Soft ride in chop. I don't think I will be traveling at high speed in 3' waves.

Stable as a old 13' boston whaler.

In terms of hp I plan on a 30hp 2 stroke tohatsu 115lbs.

Can I not use water ballast to work towards these goals. From my model I have measured I could take on between 0 - 500lbs of water ballast max.
Less load = more water, more load = less water.

I have looked at the Cootacraft little ram and it is 13' with 24 degree DR, Yes it does weight 500 lbs. Other than that not much different.
What makes the Cootacraft work, where mine will not?

 

Actually, I doubt that boat (Little Ram) is 24*, though it is certainly no less than 22*, but only the weight of it saves it from being unmanageable. But with 4 people it would probably need a minimum of 30hp, may not be legal, and would be no speed machine. The weight is quoted at 250/270 kg, which would be twice that of a bare-bones tin boat of similar size. In theory you could have a lightweight deep-vee boat with water ballast, but in practice it would be problematical, imo. Which is why you don't see them.

 

I've designed a number of tenders and these are about the most difficult things in the world to do well. This is because of the role they must play, which typically involves carting 2 or 3, often well lubricated guests back to the boat, with a few sacks of fresh groceries and resupply for the beer chest. It must do the best speed possible given widely varying conditions (crew and sea state). It has to be light so you can haul the thing aboard or up into davets and a sweet, soft ride would be nice to add to the mix. Yep, a tough set of compromises to settle on.

The extreme deadrise approach wouldn't be the way to go. You'll soften the ride a bit, though this approach is reserved for its high speed attributes, in which it's deadrise can be beneficial. The choice of a very deep V means the displacement goes up, just to get her on her preferred lines, so there goes the weight issue. Water ballast can be helpful, but you'd have to pump her out, before you can haul her up. Maybe a string of Rule 1,500 GPH pumps can do it in 10 minutes or so. Come landing on a beach, the deep V is going to be problematic.

A better approach would be a fine entry monohedron with modest deadrise, say 12 to 14 degrees, so the ride is soft, you don't pick up nearly as much displacement and she'll have better initial stability. In fact, given it's a tender, I'd recommend a warped bottom, again with modest deadrise, to improve initial stability, lower power requirements to get on plane and improve load capacity.

In fact, I can think of several designs that would do well for your needs, most in the 12' to 14' range and all about 120 pounds if made from plywood. Carbon composite (single skin or sandwich) would make it a lighter boat, though you'd have to ballast her down to get to her lines, which is self defeating. These boats would be displacement or semi displacement, capable of carrying 4, with a hefty load too. Once you ask this size boat to go much faster, the hull weight goes up exponentially, to accommodate slamming (and other) loads. Simply put, a 5 HP outboard on something like this will push you to about 15 MPH, assuming say about 500 pounds displacement.

Yes, you can use water ballast if you like, but it takes a lot of water to get a modest amount of weight. Two well fed friends take up less room than the water ballast they'd represent. Picture a box 12" wide, 24" tall and 4 feet long. This is 500 pounds of water ballast, not counting it's container. That's a huge chunk of elbow room to sacrifice in a small boat. This is why you don't see in much.

Lastly, I don't see a need for carbon composite construction, if you can live with 15 MPH (or less) speeds in reasonable weather. A simple glued lap skiff, or maybe a multi chine can come in very light, certainly inside your composite 125 pound range. I have a 9' dinghy design that is full up at 60 pounds. A bit small for your needs, but intended as a yacht tender and can carry quite a load at displacement speeds. It could be stretched to 12' and have it's after sections reworked to accommodate low plane speeds and would likely be under you 125 pound threshold. My point is, there's a fair bit of understanding necessary, to work out the multitude of compromises necessary, in a small boat. These at times can be more demanding than larger craft, because you don't have much room for error, before some other requirement falls off a cliff.

 

Mmmm, the only V hulls getting anywhere near as stable are pretty shallow angle on the deadrise (9-14 deg?) at least in my experience. However they can be reasonably behaved in moderately choppy water. Tricky compromise, because of the above wise comments.

You will also note that an 11' Whaler type is much more prone to nose pitching with the throttle than the 13' version (even with appropriate power units say 15 and 25 Hp) so the length becomes very important especially with 4 on board. This tendency is no less marked, in fact possibly more so, with a V hull, so it could be a wet ride when crossing other wakes etc. Also note that generally most 11' Whaler types have a max rating of 25 Hp which is in fact a tad high considering the modern 15 and 20 4 stroke units. The extra aft (engine) weight does not help the pitch tendency.

PAR's suggestion seems eminently suitable as a route to pursue.