Glen-L.com

Yeah, I see the diminishing returns there. But, I am still not up to the 'rated' rpm as yet. So, I should still be increasing HP along with rpm, right?

Absolutely...After max HP has been reached I suspect power will drop off as RPMs increase due to limitations in air/fuel flow, valve springs, cam design, etc come into play.

Test run on prop #5 today
#5. 16x15 = 4400rpm= 28mph. this is prop #1 reworked, to 15"
Today I went out in salt water, 20mph wind gust to 30, choppy water, seas 1-2 ft, blowing whitecaps. However, this prop didnt really seam to care if I went upwind or down, got 28mph. Into headsea or followed, 28mph. Seems to run bow a little higher, but it so choppy, it was hard to tell. Best hole shot though. But then, I didnt build this for a holeshot.

Ken,not to ask the obvious,but are you testing with full load like you would normally run?

IE a full cooler,gear & hopefully full fish box?!!

Usually,when you prop an outboard,you do it with the load & purpose that you use the boat most for.

Just wondering if the load would make a difference in your case,but it seems to run pretty much the same whatever you try.

Maybe you've reached max with that hull/engine set-up.

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

No, I'm running light right now. I am shooting for a prop that overrevs a small amount to account for load. I got another prop coming Thursday, a 15x16. I have a gut feeling the 15x16 is gonna be the sweet spot for size.
After all, this is a Lobsterboat hull form. It is not the lines of a real fast planing hull. However, it does plane and the best part I like is that it does not fall off plane. You can slow down without the bow pointing at the moon.
Getting up to speed there is no getting 'over the hump', it just continues to go faster until it is wide open.

Sorry to ask such a stupid question, but have you checked the GPS? It is very strange that your GPS gave a reading of 28 mph in both the head and tail wind situation.

Just curious though, how fast are you looking to go. Also, have you thought about going with a smaller prop? Your engine might not be up to the challenge of cranking a 14+ diameter prop around, but could be very happy with a smaller one.

On a slight tangent here, but a couple years ago a local company designed and built a high speed planing boat for the coast guard. If you were to plot resistance/power needed vs. speed, there is a hump that must be crossed (getting the boat on plane), once on plane, the power needed decrease for as speed increases. Due to incorrect weight estimate or powering miscalculation, insufficient power was installed. The max speed of the vessel was about 5 kts. Design speed was in the high 40's.

No, none of that seems to apply to me. I have previously checked my GPS against another one, it is OK. But the 16x15 prop just drove the boat its given speed no matter what. There could have been a .x varience, but speed measured in tenths aint what I'm looking for. Best speed so far is the 16x16, got real high 28 with that.
If I can find a 14" 4blade, I would try that.
The calculations for HP vs speed are correct though, but it wont swing the calculated prop. I am gradually using smaller prop to get rpm up to rated.
I would like to get 30mph WOT and 3000 cruise at 22.

Inboards with the thrust going through, or close to, the CG and with an upward component tend to "run flat". You don't tend to have as much "hole" to come out of.

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

What are some common rules of thumb for motors as far as

1. ideal RPM for best fuel conservation
2. ideal RPM for best speed (safe) with out going full throttle

Do you want the motor to be designed for the boat to run at optimum cruise speed (from a fuel conservation standpoint) at about 75% power?

I am trying to figure out because the Coastal cruiser can use a bunch of different sized motors for various top end speeds...but which one is the most efficient? If you are only going to use the full throttle top end on occasion then you must be able to calculate what the optimum motor size is for the 99% of useage?

Man, I cannot explain stuff for crap. I still want to stretch the Vandal hull to the Coastal cruiser deck and topside plans so i can have a cruising boat which is big and trailerable. (Cuz some time we run out of water around here)

Steve

Steve:
All your questions are mute until you specify "how fast' you intend to go.
I am not snubbing you, but it is fact that speed requires HP and HP equates to fuel consumption, and fuel = dollars.
The design you mentioned can run comfortable at various speed from slow to planing.
First you must tell how fast you want to go.
From there you figure in the amount of power you need to go that fast. (horsepower)
Then, you decide on the type of engine (outboard, gas, diesel, etc.)
Then you launch it and have fun.

I understand Ken, you are not snubbing me,
that is quite alright..I really do want to know how to figure this stuff out.

I wish i could skip to the " launch and have fun part" like you have reached, but I need some wood first !!...(or a plan...another plan) ..

I am wondering if (not necessarily bigger is better) like in electricity sometimes 220v makes sense over 110v and it sometime less expensive to run certain equipment. Is that the same with a boat ? Like, if you need 150HP to go 30kts, is it more rreasonable to go with a 300HP motor and run 30KTS at half throttle? (Even if the designed hull speed will not exceed 40KTS say)

I guess my best bet is to pick a design, realize that I cannot make a displacement hull become a planing hull by adding more power, and then try to figure out what size of everything I need.


I would love to build the dynojet...but I aint getting any younger and if I dont start to think about the boat I would ultimately love to have, it will be over (my life that is) before I begin ! So I want to go bigger, in the event I ever decide to run some sections of the intercoatal etc...


Steve

Sorry for rambling..that's just me!

a big engine loafing is not the most efficient. When you get to the point of actually installing the engine in your project, the engine manufacturer can provide the data you need.
Somebody once said "if your not gonna build a boat.......you may as well not build the one you want"
Same is true for the engine

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

OKay, okay, I see how it gonna be.,...the two southun boys gangin up on the poor little Yankee..you both know that aint fair !! It is gonna take at least another four or five southuners before its even !

Steve

Steve, depending on the design adding more HP to it will do nothing more then burn more fuel and cost more money. Pick your design and then go from there. If its not a planing type hull then IMHO just go with the recommended motor/HP levels...

BTW how many sets of plans do you have? I'll shortly have 3 sets and not a boat in sight YET