Custom Bream River/ Harbour Boat

[Choppy]

I dreamt, i started, now i'm commited.

Thought some of you may be interested in my little project.

I cannot get the photos on. How do i make them 500 * 500.

Thanks

[jimi]

Choppy, if you don't have any fancy software

- go to paint,
- Open image
- Go to Image->Attributes and check the pixel size. this should be less than 500x500
- If not, go to Image->Stretch/Skew and type in a stretch level like 70% or something to shrink it.
- Make sure horizontal and vertical are the same or it will be distorted.
- Check the size again in attributes, or hover the mouse over the bottom right corner to see the size in the status bar.

You can calculate the right stretch ratio beforehand pretty easily if you want to make it a bit faster (finalsize/initialsize *100). Do that for the longest size and then use the same ratio for both sides.

Cheers
James

[Choppy]

Hull

[Choppy]

Inside

[Choppy]

Transom still underconstruction

[Choppy]

Any critisism would be great. Bring on the ideas, praises, put downs. Anything. I want to hear it

[jimi]

Interesting project boat there Choppy, did you design it?

It looks like it would be mega stable for river fishing. I think it would bang a bit if heading into some steep harbour chop though as the center section is fairly flat and wide (a bit like a punt).

Also, are you having 2 outboards or one? I've heard that it is very tricky to avoid cavitation problems with a single outboard on a cat. Webster seems to have found a way around it and markham whaler avoided it all together by mounting the motor on one side (at the expense of steering problems) in their latest model though.

Cheers
James

[Evileye]

hey coppy, the boat looks narrow, what is the beam?, and the welds, just little spots, could there be a possibility that they will break in rough banging?(from the sea of course!), but it looks nice iv never seen a boat like it so finish it and tell us how she goes.

cya

[Choppy]

The design of the structure is similar to drawings which i purchsed from a designer in melbourne. The external attributes i changed some what.

The original design actually had a flat transom. the deadrise i have put in place is 15 degrees. i tried to obtain a good balance between handling chop and keeping draft to a minimum.

I will putting twin 40hp evinrudes. I was also worried about Cavitation, the original design had the motor in the middle and only a single. Thats why i'm mounting 2 (including speed and safety of two motors) and also i have moved them 450mm back from the bottom of the boat in an attenpt to get clean water.

Hope i'm right - there will have to be heaps of trial and error in motor height placement though - i just hoping i'm extremely lucky and get it right first time.

[Choppy]

The welds you see have been kept to a minimum so they are not in the way when i come along and seam weld the boat - the inside has already been completed seam welded.

I actually thought the beam was wide at 2m. In reality its looks huge due to it being that width the whole way - some what different to a mono hull.

[Evileye]

2 meters is plenty wide for a bream boat, most pics do loook decieving hay?, and holy crap, twin 40's will get it it flyin!, how big is the boat? good luck with it.

[TuckerT]

Excuse my ignorance, but what does cavitation mean and what causes it? I am not a boaty but hope to be soon and am keen to find out as much as I can.

Trev

[jimi]

Hey Trev, cavitation occurs when the prop is spinning in water that is mixed with air. That can happen due to poor hull design, engine placement or turning too tightly (shouldn't happen in a well designed hull).

When the prop encounters aerated water, it will spin too fast because there is much less resistance, and this can be dangerous for boat handling. If the motor overrevs for too long, it could damage to motor. Most motors have rev limiters to stop damage these days.

Cavitation can also cause pitting on the propeller blade surfaces due to the sudden drop in pressure that occurs.

Hope that helps

[TuckerT]

Thanks Jimi. So if I'm trialling a boat, would I be able to hear the cavitation? How can I know if cavitation occurs?

Trev

[jimi]

You will know if there is a cavitaiton problem. The motor will suddenly increase in revs (and noise) and you may loose forward momentum. It's not a continuous thing, it will usually happen suddenly when doing a tight turn or bouncing over a wave.

If the motor is too high on the transom, you may have cavitation problems. You will be able to make it cavitate more easily in that case. Of course if it is too low, you will loose top end speed, which is a harder thing to judge in a boat test.

In most well designed production boats it's not really a problem unless something has been set up wrongly.

Actually, techically speaking, cavitation may occur without you noticing it in some situations. If the prop is the wrong pitch or the motor too high, you may see streaks forming along the back of the propellor blades after a while.

These are caused by water being vapourised due to low pressure generated by the prop. This causes mechanical wear on the prop which manifests itself as pitting over time. Hence the streaks.

In that case a prop change or motor hieght adjustment may be needed. See someone like Mike at Just Propellors for more help then I'd say