Amount and placement of fins on a 18ft jonboat

[snaai]

Hi all,

I’m working on the design of a small, fast flat-bottomed workboat and I’m looking for some advice on the longitudinal fins/skegs underneath the hull.

I’m mainly trying to figure out:

• Would you recommend two or three longitudinal fins for this type of hull?

• How far apart should the two outer fins be, particularly to avoid disturbing the water flow into the propeller and causing ventilation or cavitation?

• If using three fins, could a centre fin directly ahead of the propeller negatively affect the water flow into the prop?

• How important is the shape of the fins? Do they need a tapered/triangular profile, or could a simple rectangular fin running along the hull work just as well?

• Roughly how deep and wide should these fins be for a 18ft jonboat?

• How far aft should they extend? Should they stop well before the transom/propeller, or can the outer fins continue almost to the stern?

The boat is a relatively small, fast, flat-bottomed outboard boat. The main purpose of the fins would be to improve directional stability and handling without negatively affecting performance or propeller flow.

Any experience with similar flat-bottomed boats would be very helpful.

 

[Skip Johnson]

Two should do.

 

[kapnD]

The “fins” under a flat bottomed boat are often as much for stiffening the bottom as for directional strakes, so you’ll need to evaluate how much help the bottom will need to hold its shape once the boat is loaded and underway.
Can’t really offer more than that not knowing construction details of the boat, intended power and use.

 

[montero]

What for do you think you need this ?

 

[snaai]

The “fins” under a flat bottomed boat are often as much for stiffening the bottom as for directional strakes, so you’ll need to evaluate how much help the bottom will need to hold its shape once the boat is loaded and underway.
Can’t really offer more than that not knowing construction details of the boat, intended power and use.

It's not for stiffening the bottom as it's got internal ribs. They are meant for directional stability and as bumpers for when the boat is beached.

 

[montero]

You don't need anything like that at all. Youwriting that the boat has to be "fast." This is completely unnecessary.
Additionally, it complicates construction and repairs.

 

[BoatBuildLog]

What speed range and prop height are you designing around? Seems like that would matter quite a bit before choosing between two or three skegs.

 

[HelmutSheina]

Anything in line with the prop may cause vacuum cavities on turns, which can cause erosion (or cavitation burns) on the propeller. I would go with two low strakes kept wide on the hull and terminated and tapered at least 600mm from the transom.

 

[snaai]

Anything in line with the prop may cause vacuum cavities on turns, which can cause erosion (or cavitation burns) on the propeller. I would go with two low strakes kept wide on the hull and terminated and tapered at least 600mm from the transom.

Thanks! i'll leave the middle one out then. I'll also decrease their height from 4cm to 3cm as I think that'll help a lot with reducing cavitation.