Deadrise OB15

[FrisianAngler]

I'm very intrigued and interested by the OB 15 design. But I can't find the deadrise at the transom in the description and/or studyplans. Does anyone happen to know what the deadrise is?

Many thanks in advance

[jacquesmm]

The OB15 has very classic sportfishing hull, a good all around design.
Deadrise is 15 degrees at the transom and 39 degrees at the cut water.

The deadrise at the transom is only one element of what makes a hull. It is not listed on most production boats brochures.
You can have a bad hull with a high deadrise at the transom but a poor volume distribution making it less seaworthy and riding harder than a boat with less deadrise.

[FrisianAngler]

Thank you for the fast response!

I have been reading and studying hull design frantically, and I have come to understand that it is a very complex matter. Having said that, I also read that the more deadrise a planing hull has at the transom, the softer the ride. But it also needs more power to reach the same speed as a hull with less deadrise at the transom. Since I will only be using this boat in semi-protected, fresh water, on sunny days with fairly low wind conditions, a deadrise of 15 degrees seems close to perfect for my purpose. It should get fairly easy on the plane and provide for a smooth ride with good fuel economy.

From what I understood, you are the designer of this boat. Do you think, if I built it to be tiller-steered, it would get on the plane with 2-3 adult with just a 20 hp 4-stroke outboard?

[TomW1]

Welcome FrisianAngler to the Forum. I sort of help guys work out boat speeds and then select there props on here. I worked up the OB15 with weight as per study plans, added the weight of a 20HP short shaft Yamaha motor and then added 300lbs for any and all gear and other things you will bring on board. That includes things like gas, battery, lights, safety equipment, cooler and drinks, etc. It adds up fast. I use 175lbs for adult weight.

So any way after all that the calculators show a top speed of around 24 mph. So you should be able to plane at cruise at around the mid-high teens. I also checked the Yamaha 25 and it adds 3mph in top end speed to 27mph, which will give you a higher cruise speed.

Good luck on your build and please ask if you have any questions.

Tom

[FrisianAngler]

Awesome Tom! Thanks a lot. It may take a while before I can actually start the build, but the research up front is half the fun for me. I really want to make sure this boat is the right boat for me, and from what I read so far, I think I might have hit the jackpot.

[jacquesmm]

Yes, I am the designer.
Tom's calculations are correct but keep a few things in mind.

25 HP at the shaft usually requires a 30 HP. Then, you get that 25 or 30 HP at a high RPM.
I would say, yes, a 25 will bring it on plane with the load you mention but I would feel better with a 30 that you run at 4,000 rpm.

Alone in the boat, you'll get on plane with a 15 HP.

Pay attention to weight distribution. I like the tiller steering but you can't sit all the way in the back and expect the boat to get easily on plane. You will need a tiller extension when single handed. Those extensions are not expensive. No problem with a crew.

[FrisianAngler]

Thank you Jacques, for the clarification. I will be alone in the boat most of the time, so 20 HP should suffice 95% of the time.

I bought the plans, but I'm still debating whether to use tiller-steering (less fuss with cables and more room inside the boat) or to build a console in it. Forum member R. Torres built one with a centre console, and his boat looked awesome! It also seems to help to centre the weight distrubution. I think he also put a 30 HP 4-stroke Suzuki on the transom. I still have plenty of time to decide, since I won't be starting to build very soon anyway. But when I do, I'd like to keep track of the progress and will post on the forum.

Thanks for all the help and information so far!

[jacquesmm]

I just noticed your user name: Frisian.
I have good memories from Friesland in particular the islands: Terschelling, Ameland, Vlieland.
When you say protected waters, is that the Ijzelmeer?

[FrisianAngler]

The protected water I meant, are the lakes and canals around the town where I live: Sneek. The IJsselmeer is a lot bigger. Strictly speaking, a smaller boat would probably be more than enough for "my" waters, although the lake can be quite choppy. So suprising that you're familiar wwith the islands likeTerschelling! I love Terschelling! With all of the Corona lockdown measures and such, we haven't been there for a while, but it's awesome!

[jacquesmm]

A long time ago, when I was a student, I went camping on Terschelling. At that time, we called the IJzelmeer ZuiderZee.
Later, I sailed around there. We had to watch the tides!
For the canals and inland lakes, the OB15 is perfect. In go weather, you can go out everywhere in that area.