Determining Hull Type;

Hi,

I have a small motor boat 18ft which I guess is 30 years old and had an old MD2B 20hp volvo penta in it. I am guessing it was a small commercially used boat from a port in south of france. In any case I have it dry docked now and am installing a new Yanmar 3YM20 (20Hp) engine. What I am trying to determine is whether she is a fully displaced Hull or semi displaced hull as I am hoping to get more speed from the new engine. I have photo's of the hull and finding it difficult on the net to get a definitive answer. If she' semi displacement then I assume I should get some improvement of speed which I estimate with original engine (also 30 yrs old) was around 5-6 knots.

If it's a displacement boat I realise Im restricted to 1.34 time the root of the LWL, if not and it's semi displacement, what speed would the new engine generate ?

Should I consider putting in a larger prop with new engine, not looking to be a speed merchant but if I could get her to 10 knots that would be brilliant?

Would appreciate and advise or referenece to sites which have pictorial representations of fully displaced Versus semi displaced Hulls.

 

It's difficult to say anything if you don't post some photos of your boat here...

 

Photos now attached, thanks

 

Do you have a profile view?

 

Took these yesterday on iphone no flash, daylight fading will try to get a better shot with flash. That aside what would your guess be? The last photo shows some profile but shawdowed I accept. Many Thanks.

 

The chined hull, plus the bottom with what (from the first photo) looks like a big rocker, plus very shallow aft deadrise makes me think that this in fact is a semi-displacement hull.
Assuming that 18 ft is LOA, and that LWL is some 16 ft, The maximum practical displacement speed will be some 5-5.5 kts, and I believe that you should have no problems at attaining some 8-8.5 kts. The displacement and weight balance (CG position) are important parameters, especially with small and light boats like this one.
10 kts is a bit streched, imho. It's almost a planing speed for this hull, but it is not (apparently) made to plane safely and efficiently, mainly because of the rocker. And would probably require lots of power (means more engine+gear weight!) to do so. But before being certain about that, I would need to see the profile photo.

 

Many thanks for this, that is quite helpful, Im away for a few days but will get some better and clearer shots. Even attaining 8 knots would be fantastic. I am new to boating, will look , so not to sure what a rocker is but will look it up. The guy putting in the engine tells me I need a new prop as it's counter clockwise to the config of the new engine. From the shot of the prop (I know hard to see from shot 3 but I'm maxed out on my upload quota) , do you think I should order a larger or more pitched prop from the original. In any case thanks for your input it's much appreciate .

 

Without knowing (at least) the boat's operative weight, waterline length and beam, actual propeller's diameter and pitch and engine's actual gear ratio, it is really hard (read: impossible) to say anything from the photos.
Apart that you will very most probably have to change the prop if you want to arrive at 8 kts.

The rocker is a therm indicating the curvature of the keel. The belly...
Displacement hulls generally have a nice rounded keels, planing boats have it flat. Semi's are somewhere in between.

This one has a rocker, for example:


This one doesn't:


Do you notice the difference?

 

Rocker means that the draft reduces toward transom thus the hull is not a prismatic hull with flat bottom made for planning speeds. It looks to me that there is no rocker, but it is very difficult to see from the photos. Rocker helps at lower speeds. Any idea about weight?

Since you already had a 20 hp motor, it would help much if you provided all the data for that. What was the propeller size, gear ratio and the max rpm? If the propeller was even close to OK, why would you have more speed with a new 20 hp motor? Or was there something clearly wrong in the old one?

Joakim

 

The first photo, it looks to me like the hull bottom curves upwards towards the transom... But without a profile view, it's uncertain. Maybe the boat is pitched down.

 

Thanks J.

From pictures D. posted I don't believe there is a rocker on the boat. I realise I need to get a bit more data. The old engine is twice the weight of the new engine (500lbs versus about 260 Lbs). + as a 30 yr old engine not sure the output would be really 20 HP at that stage in it's life. The new gear ratio is reduction of 3.22 generating max 16 kW at 22mph @ 3600 rpm. My problem is I don't really know what the old ratio was , the max rpm was around 1800 rpm which got her to about 6 knots or so. I'll measure the prop and gather a bit more detail on the dimensions as well and perhaps put a question under the propulsion forum.

Daiquiri , I'll get some better pictures next week as I'd like to know fundamentally what hull she has, thanks again. Appreciate these are quite novice questions
but Im new to this and the engine I put in is worth more than the boat so I want to get the optimium configuration for engine and prop.

Paul

 

20 horses will not get this boat onto plane no matter what shape the bottom is. I can't see that there is any bottom rocker from the photos but there is definitely some rise in the bottom toward the transom. Enough power would surely move this boat well into the semi-displacement mode and it might even plane, provided the aft buttocks are straight enough. A buttock is a line drawn on the hull surface by a vertical plane parallel to the hull centerline. "Buttocks" define the longitudinal shape of the bottom. If they have convex curvature, there is rocker which largely determines the speed potential of the boat.

20 hp is not enough power to drive this boat very far beyond hull speed though and the 8 to 8 1/2kts that Daquiri mentioned is probably pretty close.

 

How do you know the old motor was 20 hp? There seems to be 25/27 hp motors at that model: http://hem.passagen.se/eksnipan/Eks...Datablad 1974 Engelska 74.13201 ref.6087C.pdf

Since you only got 1800 rpm at which the motor above already gives ~almost 20 hp, you were most likely not rpm limited by governor, which would be an easy reason for low speed.

I wouldn't be surprised, if the top speed stays close to 6 kn.

Joakim

 

Thanks Tom, that's useful, I'll post a clearer picture tomorrow if I can get out to the boat which will make it much easier to read.

 

That's a full plane hull if I've ever seen one before. Her run looks clean and straight (mostly), her transom is more deeply immersed then a semi displacement hull form, she's also carrying reversed curve strakes, which a semi displacement hull wouldn't likely need. What I see is a warped bottom form, with moderate deadrise.

A 20 HP straight shaft will drive this boat to about 6 MPH. A 70 HP engine will lift her up on plane, with a 100 HP engine offering fairly good performance and a healthy reserve. You could toss 200 HP at her, but you'd run into a wall of limited returns at about 35 MPH.

She looks to be a reasonably dry running boat and able to tolerate moderate seas, before getting too uncomfortable. This would make a nice stable fishing platform for rivers, bays and near shore deep water work, with occasional blast into blue water in nice weather.