First time boat-builder

[lazeyjack]

i tried assist you by pm, never got acknowledgement, so

[timplett]

Lazeyjack,

Sorry about that. I read your PMs and they were rather helpful, I just completely forgot to email back. I've been sick, not sleeping well, and busy with work so my mind isn't quite here. I am sorry I never thanked you.

Tim Plett

[lewisboats]

An observation I will make is that you are likely to trip on that chine and that would be ugly. I think you will be really nose up too, until you get to a S/L ratio of about 4:1 and you are on the planing pads properly. You have an awful lot of beam for the hull style and length...perhaps you should consider narrowing it a bit, if only to reduce friction.

Steve

[VKRUE]

Reguarding; (My idea was mainly based on the fact that I would have access to a bunch of free steel)

Not to be rude but... so what ! I have access to all the FREE concrete that I want but do yu think that It is useful in building a boat!

Listen to what the MANY people around here are trying (nicely) to tell you...

Sometimes, I think that someone just sits around an dreams up this stuff to get everybody stirred up.
By the way, about that surplus of free concrete................

[timplett]

lewisboats,

The reason for the wide beam is to fit two people side by side abaft. The rest of the hull would then be covered. I had considered making it a one-seater, but what fun is a boat with no one to share it with? Why not make it longer then? More material, more work, more money. About being nose up, Free!Ship calculated the longitudinal centre of flotation at roughly 3.3 feet forward of the transom, so would that not make for a nose down stance (unloaded)? The larger flatter section at the rear should provide more bouyancy than the front should it not? The outboard would then tip the nose up, but I had planned for a fuel cell and battery in the bow, which would hopefully help balance this. I'm doing my best to figure this out, but a little guidance is needed and much appreciated.

VKRUE,

What I was saying is that the free steel is why I came up with the idea, but I changed my mind after some input. And during WWII there was a design for aircraft carriers built out of something called Pykrete, a concrete-like mix consisting largely of ice, but I digress... I think it would be possible to build a concrete ship, just extremely impracticle, but if you've ever got some of that free concrete and some free time, give it a whirl.

Tim Plett

[timplett]

lewisboats,

I have done some more follow up reading on the LCB and LCF. As far as I can tell, the LCB is similar to a fulcrum upon which the hull balances (to reduce this to simple terms). The LCF if the centre of all the weight. Whichever side of the fulcrum the centre of the weight is, that is the way it tips. The hydrostatic calculations in Free!Ship place the LCF just slightly forward of the LCB, promoting a downward tilt. The calculations from 0.1' - 1' draft show that as the boat sits deeper, the LCB moves forward some, but the LCF moves further, to roughly 6" forward of the LCB (as far as i can tell, fairly negligible). These calculations are all with 0 trim, but the LCB and LCF stay within roughly 8" with +/- 1' of trim. Once again, I'm new at this, and interpreting best I can, but as far as I can tell it seems fairly balanced. As I said though, help is greatly appreciated.

Tim Plett

[antonfourie]

I see from your design that your displacement would work out to 189kg, no chance of it ever being that light with steel and concrete.

Best to google "stitch and glue" for a small boat like this ....

[timplett]

Just for the record, I decided to make it a plywood-core fiberglass design. This might not have been made clear with all the bantering back and forth.

Also, Regarding my last post addressed to lewisboats, I realized that I confused the LCF with the LCG. I'm having a little trouble keeping track of all the new stuff I've been taking in. In correction to my previous post, using the LCB and LCF provided by Free!Ship, and calculations I found online, I found that I would have a trim of -6" at the stern, so yes, nose (bow) up. What I am wondering, is if it is best to try and correct this by changing the design, or just by changing the weight distribution in the final product? How would I have to modify the design to change the trim of the hull? I have also looked into the benefits of trim tabs but I think that might be a little overkill on a small boat like this.

Tim Plett

[lewisboats]

You wil have to move the LCG further forward. Move the cockpit a bit forward, put the fuel tank up front (don't forget all the venting and safety concerns). You will still probably need some kind of trim tab or trimable mount for the motor. You could also design some sponsons around the motor to shift the LCB aft as compared to the LCG.

Steve

[timplett]

So if I basically make an "indent" in the transom to move the outboard forward, say, a foot, that would help make the boat more level?

Tim Plett

[timplett]

Modified the transom to move the outboard forward a foot. The LCF is now abaft of the LCB, which should actually made the nose dip down a little (before load is added), right? New files are attached.

Tim Plett

[marshmat]

Hi Tim,
Firstly, I think you're starting to find an OK direction for this thing. What you have bears a vague resemblance to a Stock Outboard runabout, a fairly common teenager's racer.
The indent in the transom, I think, is not a particularly good idea. This will significantly reduce buoyancy and lift in the stern (not good) and be harder to build (also not good) compared to a flat transom. With little zippy boats, you have to accept that there will be a nose-up attitude at rest and the crew position will be what determines the hull trim. You're essentially turning it into an 8.5 foot boat by making that cutout, as the part protruding past the motor will not add much buoyancy at all.
I would be quite worried about tripping on those sharp chines at the stern. Most fast outboard boats this size have the hullsides flaring outwards as they rise above the waterline; tapering them inboard like that tends to encourage the chine to dig in and roll the boat in turns. Also, if you're going with a warped-V (deadrise not constant) as you have drawn, deadrise should reach its minimum at the transom, not at midships. As drawn, with the V sharpening aft of midships, the stern will tend to squat at speed because there's too much lift forward.
As to the choice of power. I can absolutely positively guarantee that there is no way the Coast Guard will give you a capacity plate for more than 20 hp on that boat; it simply doesn't have enough size to be stable with any more.

[timplett]

Well I must laugh at my ignorance. When someone else suggested that I would trip on the chines, not knowing what they meant, I assumed they must mean when I was boarding the boat or something like that... I see the logic in that now. I am going to be honest and say I incorporated those chines because I like the looks, and that eventuality never occured to me. Flaring the hullsides outwards may also allow me to narrow the beam (as someone else suggested) and still fit two people side by side.

As for the deadrise, I checked my drawings again and the minimum deadrise is at the stern, or more specifically the whole section between the stern and two feet forward of the stern. Maybe I am calculating something wrong. By looking at it looked as if you were right, but calculating the angles showed that the transom had the least deadrise. I calculated by dividing the difference in height from the keel to the chine (at the transom) by the horizontal difference between them (simply, rise over run) and multiplying by 45 to get the degree value. I repeated this every six inches or so forwards.

About the power. Is the number on the capacity plate a law with penalties for breaking it, or is it basically a suggestion to cover all parties involved when some idiot puts a 200 HP outboard on his 8' dinghy? Not asking because I want to joint he ranks of the idiot that would really overpower his boat, but just in the eventuality that it does get rated at 20 HP and say I find a good deal on a 25 HP motor or something like that, can I legally put it on? My other concern is whether it will be fast enough to be fun. I don't mean stupid, death-defying fast, but 10 HP, no matter what it's on is only exciting for the fact that you don't have to row.

Concerning the crew positioning, any position forward of the LCF will help hold the bow down right? Obviously further forward will have more effect, but I am wanting to have the seat fairly low down, requiring a little more leg room, so I don't want the crew too far forward. Once again, I am a little concerned about looks too, I like the looks of the crew further back, but not so concerned that I will sacrifice other design elements like performance and stability just to get the looks I want (example: I'm changing the chines, which were basically for looks, to avoid tripping).

Have you ever registered a home-built boat in Canada? I have been doing a lot of research, and the matter of the HIN has got me stumped. I can't find where to I will have to (eventually) apply for one. Transport Canada refers me to Service Canada, which directs me to the Office of Boating Safety which refers me back to Transport Canada.

Thanks for all your input,

Tim Plett

[marshmat]

Class A stock outboards routinely top 40 mph with a 15hp, and they're not much smaller or lighter than your plan. A 40hp on that thing will flip it nose-over-tail quite easily; a 20 would be enough to get most passengers decently white-knuckled. The deciding factors of the calculation are, simply put, the boat's ability to safely handle the weight of the motor, and its ability to maintain a straight and level course with the motor at WOT. Overpower at your own risk. (In my case, I was young and inexperiened when I got the plate for my first boat, and so measured it wrong on the forms, thus giving a plate rating 30% lower than it actually should be. The current motor is sized for the correctly done calculation but I haven't bothered getting the plate fixed... don't tell Ike )

Straightening out your hull model seems to indicate that my comment on the transom deadrise was due to the weird cutout and not due to the actual hullform... although the statement holds true your model isn't actually at fault on this count. Do fair the chines and keel though; weird curvature inflections in the chines will cause weird handling quirks. For reference, deadrise = arctan ( delta Z / delta Y ) but keep the lines fair and smooth and you'll only need to worry about deadrise at a couple of points (transom, midships, bow, that's about it for a boat like that).

As far as I know the procedure for getting the government paperwork hasn't changed since I launched Sunset Chaser in 2001. Read the small vessel construction standards (request document TP1332: 1999 from Transport Canada, I'm pretty sure it's free), fill out a form from the back of that book, send it in with a cheque and photos. You then get a plate in the mail that gives a HIN and weight, passenger and power ratings for the boat; you stick it beside the motor mount. Then you will get a letter from the Ministry of Finance asking you to pay the PST on the boat; you write back saying "I built it myself and bought all the supplies legally, so piss off now" and you're good to go. If you want more than 10 hp you also need to get a registration number to paint on the bow (free, takes 10 minutes).

[timplett]

Well it sounds like 20 HP will be plenty enough to keep me happy. The other benefit is that the 20 HP will be a fair bit lighter than a 40 HP, hopefully helping with the nose not sticking up too high. Also would it make sense to slope the top of the bow down slightly, I believe it is called downward sheer (I was just reading about that in another forum), or is the boat too small to bother?

About the paperwork, what you are saying is that when I apply for the compliance label I also get a HIN number?

Thanks,

Tim Plett