4,4m OB design project

Having eventually decided to design my 2nd boat project, an epoxy strip plank outboard boat with 20-25hp engine, myself I would now hope for guidance so that the result will not be a total disaster.

After studying books by Larsson and Gerr the main dimensions became Lh 4,40m, Bh 1,50m and Mlcc 330kg. The intended use is a fly fishing or runabout boat for 2 persons and 4 persons max and target speed 25kn with one person.

The question I am considering are centers of boyancy and gravity, the hull form at this point has LCB at around 39% DWL and preliminary calculations show that VCG should settle below 0,30, meaning 0,15m above DWL or 0,20 above VCB. With these FreeShip calculations would show maximum righting moment at 30-40 degrees.

Assuming reasonable prop. efficiency Savitsky shows that 20hp should be enough to reach 25kn, or atleast close.

Any ideas about the centers and stability? Would the boat become likely reasonable or should I work to change the centers or lines before proceeding further?

 

Looks like the Stability would be good but at high speed turns these things go all over the place since there is no hull lines at the bottom for the boat to grab the water so it just slides like a rock plus you dont have a V hull which will cause your boat have a rought ride on choppy water. And if you want the max speed the back should be the same lenght as the middle. But it is up to you. You got a good vintage deisgn too. I just wanted you to know what you are getting.

 

The lines look good for calm water operation and I doubt if stability will be an issue. The hull might evolve a little as you look at the construction detail and planking. You may need to be careful with the location of the planks to achieve the flare in the bow and chine detail near the bow.

You may be asking a bit much for a typical outboard prop. I would work on 65% efficiency unless you do something special with the prop. If you go to largest available diameter prop that will fit and pitch it up for light load you might get close with 25HP. Remember the leg of the outboard adds a good amount of drag as well.

You should do some detail on the outboard well and check the transom angle is enough to allow the prop shaft to get at least to horizontal.

Rick W

 

Thank you both for your input - Sounds very much like I should have a reasonable base to proceed with.

The intended cruise speed is only 15kn so I am trying to emphasize low speed planing. Also the operationals wates will be inshore and protected coastal waters of quite dualistic essence. Either the waters are calm without any significant waves or the waves are steep and 50-100cm in height meaning that a boat of this size has to slow down in any case.

I planned to design the outboard well, finalize fairing the lines and main structural members as the next iteration. Sounds like a 25hp engine will be a wiser choice as they are also available as EFI-models, or the new little e-tec could be an interesting option.

 

You should'nt have any problems with your design then. For 4.4 m boat 25hp outboard will get you anywhere fast especially in calm waters. Don't believe everything E-Tech says these days especially the maintanence. Good luck.

 

I did some calculations with my Savitsky program including drag for drive, aero and whisker spray (without spray rails). I used 1.2 m beam (transom), 11 degree V, 400 kg displacement, 65% propulsion efficiency and quite smooth bottom.

At 25 kn the power requirement is about 18 kW, if you have optimal trim angle, which requires a LCG of roughly 1.4 m (porpoising limit). If LCG is at 2 m, the power jumps to about 22 kW.

Introducing efficient spray rails would reduce both by a bit over 2 kW.

At 15 kn the power requirement should be 7 kW with both LCG values and the effect of whisker spray is only 0.2 kW.

With spray rails and optimal trim (LCG) a modern 20 hp should be enough to reach close to 25 kn. There are boats that can do this in your size and weight, but also there are many that do only 20 kn and I think the main reason is having LCG too far forward (long WL, big wetted area).

Joakim

 

Again, thank you very much for your efforts! I had a tad optimistic view on propeller efficiency. Also I used maximum chine width while the chine width is somewhat narrower at transom. I will consider the engine selection later with better figures.

I think that a flatter surface is able to resist porpoising with LCG closer to transom than a hull with more deadrise, is that correct? A quick estimate with my preliminary weights shows LCG at around 1,40-1,50m with one person.

Short shaft engines in the 20-25hp range seldom have power tilt which gives a quick way to stop porpoising under way.

 

No, it's just vice versa. Flat bottom is the most prone to porpoising and is thus not used for higher speeds. V-bottom is more efficient, since you can have higher trim angle and thus less wetted area without porpoising. You can find the curves from e.g. the original Savitsky paper
http://raritan.dl.stevens-tech.edu/personal/mdelorme/OE 524/Savitsky_64.pdf
or a copy of the curves from this Excel calculator: http://www.boatdesign.net/forums/downloads.php?do=file&id=26

Here is also some information about porpoising (and much else):
http://www.ivt.ntnu.no/imt/courses/tmr4217/lectures/Planing vessel lecture.pdf

I don't know which chine width should be used, since the Savitsky model assumes constant width. Also I don't know the actual efficiency of any propeller that comes with an outboard. I hope and think it should not be under 65% with such a normal operating condition as yours. It could even be well over 70%, but I wouldn't count on that unless I had some real data. For a Wageningen B-Screw series propeller the efficiency would be around 75%.

Also you don't know the actual power of the motor. Quite often it is 5-10% more than the specs, even at the propeller shaft.

Thus using 65% and the spec power gives you some margin.

If you are putting spray rails, here is a design guide for them:
http://www.sname.org/sections/new_york/Whisker Spray Savitsky 3_2006.pdf

Joakim

 

Fishing seasons have passed, and things have lived a bit. LOA set to 4,5m for LB 3, and hull construction for stich-and-glue with laid deck, wooden covering boards and coamings. Other expectations have also been lowered, the goal is to power her with a 10hp outboard and hopefully hit 10kn with 3 persons totalling a displacement of 440kg. The lines shown are for the hull I am refining for a light displacement of 200kg.