semi displacement

there has been a lot of talk on different threads about repowering planing hulls with smaller diesel engines . the general opinion i have received concerning my boat is that it will just dig a hole above 6 knots. the latest question i have is. could i add a swim platform around a meter long but follow the hull shape underneath and raise it towards the stern. so in effect i get a longer waterline and the slight taper upwards should be similar to a semi displacement hull. i would also add a keel and single engine. what speed would i expect with a 75 hp and 2-1 box, propped to suit. is a 10 knot cruise possible. boat is 26 ft , 9 ft beam, 3.5 to 4 metric ton.

 

A 26' sailboat with a 10HP will do 6 knots. The power to go over that raises exponentially. Fairing the stern will make a huge improvement at those speeds. It is difficult to estimate a cruising speed without analyzing the whole design. I would guess at closer to 8 knots.

 

do you know of any of online performance calculators that are accurate . i have tried a couple in the past but they were very crude .

 

I am quite interested in this topic. Would you mind pointing me to some of these discussions?

 

Assuming a displacement hull form, with fairly steep buttock angles and a 24' LWL, you'll pull 6.6 knots (7.6 MPH) at hull speed (S/L 1.35). If you add the 39" (1 meter) fairing, following the buttock angles of the bottom, you're new speed will be 7 knots (8 MPH). So, for all you efforts you've gained less then a 1/2" a knot in boat speed, congratulations.

Without an idea what your boat is shaped like, anything further is just a crap shoot and pure speculation. In a nut shell, you need both hull shape and power to get to speeds over displacement mode. Some hulls you need a 1,500 HP turbo fan to get to ten knots while others just 100 HP. Without pictures and/or lines anything else is just a guess.

 

it is a planing hull, i will take pics next time i slip it.

 

just search through the propulsion forums, there are some good threads on the topic.

 

If it's a planning hull, then it's just a matter of power available. What are the general dimensions and specifications for the boat. A reasonable top speed estimation can be made.

 

If I remember planing hulls generally get up on plane at about 2.5 times square root of the hull length or something like that. So actually calculating when it gets up on plane is pretty easy. Dynamic stability at higher speeds is a whole other question.

 

can you go off the first post, i put the info on.

 

Au contraire.... The point at which a vessels begins to plane is one of the most hotly debated aspects of powerboat design.
Also, that speed is the result of any number of variables - shape, weight, Waterplane area, centre of gravity / buoyancy, etc, etc, etc....
On the upside, there are a number of different ways of predicting performance for a planing hull, and though often derided by the "design community elite" as being too simplistic, even the most basic method's like Crouch can yield resonably accurate results.
Like most things however, they rely on accurate information... garbage in, garbage out....

 

No, the information you've posted (in this thread) doesn't include any of the data I'd use to calculate speed. Provide the general specs; weight, engine output, engine configuration, general boat dimensions, general hull shape, deadrise, etc.

I consider 2.5 S/L the very lower edge of full plane mode, though most powerboats are still bow up at this speed. At 3.0 S/L, they've usually leveled off and are fully up on plane. This is where most consider full plane starts.

 

w/line 25 feet, beam , 9 feet, draught 2, feet, moderate v , dead rise even bow to stern, weight 3.5 to 4 metric ton. 2 318 chryslers , shaft drive 1-1 . 25 to 30 knot top speed. what i want to know is if i convert to 75 hp diesel 2-1 box and keel deep enough to protect prop and hull extension 1 mt. can i get a 10 knot cruise in a semi displacement mode. i think it should be possible. i know you designers think semi displacement is a dirty word but how else can you achieve this speed range on a boat this length. it is just an idea i am tossing around, if i decide to keep the boat long term i might do it, i already have a good diesel. i don't know what degree v it is but i would class it as moderate. around 16 to 18 degrees.

 

If she was about 5ft wide, and with the stern fairing yes.

http://www.boatdesign.net/forums/boat-design/low-length-displacement-hulls-37159.html

http://www.boatdesign.net/forums/boat-design/semi-displacement-33509.html

 

Assuming no slip, you'll manage about 12 MPH (10.2 knots) with 75 HP at that weight and I assumed 12 degrees of deadrise and a moderately draggy hull. You will not have any reserve, so a contrary current and a head wind will slow you down rapidly.