Best size fiberglass boat length and width for using a 200lb.engine 65hp.

[Papee]

This is why I started this site, some of the general rules for the big boats just don't work on a mini. A wide hull is only good for one thing, riding dry land. On a mini after you go wider than a five foot bottom you are making more contact with the water or wetted surface. The more wetted surface you have the more drag you have which means more thrust to move the boat. If you want to run some dry ground it is a trade off, the boat will be slower in the water.

You do want to have enough floatation so the hull does not sit low in the water. The lower it sits in the water the more thrust it takes to get it on plane.

I'm new here myself and currently 3/4's done my first build. When talking to a prop salesman from Sensenich in Florida, about his boat, he stressed the importance of making the hull as wide as possible. When coming onto plane (i'm assuming this is the same as "over the hump") the increased rear surface area will decrease the downward pressure in the water resulting a faster time to plane. Does that make sense?

[dr.peck76]

Papee, Is there some etiquette on just right out asking others what they've built? I mean, I'm not trying to be a smart a** here but would some take offence to that? I'm not sure how closely guarded the hull design secret is. In the drag racing world no one ever gives you a straight answer and it's kinda taboo to ask but that's strict competition for money in most cases.....I'm just throwing it out there.

[Corky]

Papee, Is there some etiquette on just right out asking others what they've built? I mean, I'm not trying to be a smart a** here but would some take offence to that? I'm not sure how closely guarded the hull design secret is. In the drag racing world no one ever gives you a straight answer and it's kinda taboo to ask but that's strict competition for money in most cases.....I'm just throwing it out there.

I don't think there'd be any problem with asking others what they've built...It's the hobby/pleasure oriented guys that would probably love to share what they've done...What worked and what did'nt work...It's the guys that want to make a living off building them you'll have trouble getting any facts and figures out of...

[Thai Sean]

This is why I started this site, some of the general rules for the big boats just don't work on a mini. A wide hull is only good for one thing, riding dry land. On a mini after you go wider than a five foot bottom you are making more contact with the water or wetted surface. The more wetted surface you have the more drag you have which means more thrust to move the boat. If you want to run some dry ground it is a trade off, the boat will be slower in the water.

You do want to have enough floatation so the hull does not sit low in the water. The lower it sits in the water the more thrust it takes to get it on plane.

Hey Papee,

I did not realize you were the site owner, you are so modest. Well I am here so the site works. I have no one to talk with here in Thailand and essentially no airboats around. This site and you have got my interest and ambition growing. I lost my Thai mentor a while back due to cancer and all I have now is a sagging confidence.

Have you ever thought about writing articles here, that could really start things going even better than it already is, perhaps an introductory article describing the differences you mentioned in you last post here. I’m thinking this post will be interesting to future builders.

I build long tail planing boats and mine are faster than the Thai canoe and going all the shallowest rivers here, so that is why I am into airboats.

I don’t want to go on dry land, just shallow sandy skinny water and some wet grassland. I was very worried about a 6 foot width and the wetted surface drag issue. It was my plan to go 5 by 11.5 and use that 65hp. 200lb. engine and a 54 in prop that is matched properly to that engine. That leaves 6 in wiggle space, a 3 in. port and starboard clearance. I do need a two seater. Hull weight no more than 200 lbs. Now does that make sense to you? I do want to go in skinny water and grass but no dry land. What about going even shorter, I notice the mini pro is shorter and that would mean lighter, I have no idea how wide the mini pro is and can’t find any info on that. What do you think of my dimensions and what length sounds good to you given these parameters?

Didn’t sleep well last night, I was thinking too hard on this but this morning my spirits rose when I went to the site.

Thanks so much,

Thai Sean

[Papee]

Papee, Is there some etiquette on just right out asking others what they've built? I mean, I'm not trying to be a smart a** here but would some take offence to that? I'm not sure how closely guarded the hull design secret is. In the drag racing world no one ever gives you a straight answer and it's kinda taboo to ask but that's strict competition for money in most cases.....I'm just throwing it out there.

Most of the guys here are in the process of building a boat or have already built a boat. The guys here are great and will answer any questions you have.

All i I ask of anyone is that they look through the site, build threads and do a little searching before asking questions. Many of the questions folks ask have been answered before. Many times there are specific questions that need answers though.

[Thai Sean]

I only run a 52" prop so you could spin it pretty fast. the prop is good to 3100 rpm.

Papee,

Yup that's looking good on the low side. .629. (mach 1) 1.0 is mach 1 the speed of sound. I would think you could rev even more. 4000 rpm. is 8.1. However that is a leading edge call as leading edge damage can occur the closer to mach one you get. You are in the safety zone, looks like the sweet spot...

This formula is for direct drive, reduction formula is the later one.

* Tip speed formula

To calculate your tip speeds use the following formula:
Diameter in inches x RPM's ÷ 256,000 = mach

Example: 82" prop x 2000 RPM's ÷ 256,000 = 641 fps.
1000 fps. is mach 1 the speed of sound.


Approaching .8 mach should you get there without prop damage you will start losing prop efficiency meaning loss of power and a loud noise progressively occurs prior to prop destruction. The longer the prop the slower it must rotate. The stress factors that cause a prop to develop excessive wear or even destruct vary according to prop quality and strength, you can damage a prop even if you are not that close to mach. With wood props some experts recommend to not exceed 675 fps.

* Pulley reduction formula Circumference of a selected engine pulley divided by the circumference of a selected prop pulley times the engine rpm equals the prop rpm.

The accurate way to measure a pulley is to measure its circumference and make a pulley engine ratio from that figure using Pi. Pi is the diameter of a pulley times 3.14. As an example a 2 in. diameter pulley has a circumference of 6.28 and a 4 in pulley has a circumference of 12.56. No one uses those sizes though...

Thai sean

[Papee]

There just is no one answer to the hull question. It all depends on what you are going to use the boat for. Some are just building a little ride boat to scoot around on. These boats are usually short and light. Others like me want a multipurpose boat for fishing as well as hunting and other activities. This requires a longer boat to haul a passenger and have room to bring gear as well as move around the boat while fishing.

My personal opinion is if you are building a two person boat it should be no shorter than 12' with a 5' bottom. What the length does is allow the weight to distributed over the boat so the hull sits even in the water. The more water the boat takes to float the more thrust it takes to get on plane.

I started writing a book a few years ago and it's just turned out to be a bunch of notes so far. I get writing on a subject and it pours over into another area so often. It's just so hard for me to stick to one thought when so many things with these boats depend on each other. Then there's the "perfect hull" deal, there just isn't an answer to that with the different uses and motor combinations.

These minis are not like the big boats, you just can't build any hull then just throw a bunch of horse power at it and make it work. The most important thing is wetted sureface vs thrust.

[Thai Sean]

Yea Papee,

Bingo!

Thanks I got it and I am committed, will revise my drawings now.

This all adds up with my other boats draft and weight ratio.

Thai Sean

[dr.peck76]

I'd love to see a sketch if you didn't mind Sean. What kind of gunwale height are you thinking?

Yea Papee,

Bingo!

Thanks I got it and I am committed, will revise my drawings now.

This all adds up with my other boats draft and weight ratio.

Thai Sean

[Papee]

You could spin the prop faster but because of the design of the prop it does no good. The prop will just stop pushing at a certain rpm.

[Thai Sean]

Sure,

When I am confident in its looks I will post it, I do computer mock ups. I don't really make detailed drawings too much anymore.

I'm thinking sixteen in.

Thanks again,

Thai Sean

[Papee]

Again this is all for each persons use. My boat has 18" gunnels and I wish at times they were a bit higher. I get into some pretty swift Rapids on the river here though. I think there is a video in the video section. The boat I finished last year for my uncle has 20" sides.

[Thai Sean]

If it ain't broke it don't need fixin..

[Thai Sean]

My longtail has 18 and I never get water in the boat. I run some rapids so I figured I could even go less. thinking weight. good point though with the mold and all...

[dr.peck76]

I'm a little higher than that at 24". I guess I'm a little more conservative and I'm thinking I may have over built a little after reading here. Oh well, better safe than swimming.

[txriverrat]

on the flounder rig I have 16 inch side ,that is just a little low . with a 60 inch wide sheet if you use 20 inches for sides and transom it works out with no scrap.
Ron

[Thai Sean]

I'll compromise a 18, he,he...

[Papee]

on the flounder rig I have 16 inch side ,that is just a little low . with a 60 inch wide sheet if you use 20 inches for sides and transom it works out with no scrap.
Ron

He's getting info together to make a mold for a fiberglass hull Ron.

[aerokirk]

Thai ... in answer to your questions, my Fiberglass hull measures 5'5" across the bottom to the chine break (soft chine); 6'6" gunwale to gunwale; 18" transom height and 16" side height.

BTW: It looks like you may have started two threads on the same subject.

Here's some pictures:

[Thai Sean]

Yes, two threads same subject. My mistake, I thought the post was in the wrong place in the newbee section and wouldn't get noticed by the big boys.... In Retrospect I think the new members plase will be good for new members to see, I have got some good advice that will help others.

Thanks for the pics, I hope no one minds your devulging to secret info, he,eh...

Ok now I see that hull has what old school boys call a modified soft chine, its characteristics are that in turns the hull rocks over and the inverted short section there acts like a hard chine and somewhat digs in helping out on the turn. It’s like getting the best of two worlds kinda, nice hull I can see why lots of people like it. I don’t figure I will make mine with that chine though it sits too low in the water for me. Anything more than 2 in.draft I don’t usually use. Hey next time it is in the water could you measure the draft at the middle of the transom with two passengers and normal gear for me, I am really curious… Im not really a soft chine guy but I admire them...

Just looked at the pickof your rig floating, looks plenty high in the water...