correct power

[sonkilli]

Hi there,<br /><br />i have bought myself a new boat, and as im new to boats of this size i have a question.<br /><br />I have a boat with cabin 7 meters long and 2,45 meters wide. It has gross weight of 1200 kg.<br />It has a hull of a speedboat.<br /><br />How many HP do i need to run it with optimal fuel consumption to speed ratio.<br /><br />Or perhaps is there some formula that will let me compute this. I have stumbled on couple of such formulas but they were all designed for smaller boats than mine.<br /><br />Thank you<br /><br />Sonkilli

 

[vidar]

Re: correct power

what is the name of the boat? type and producer.. but a 225 pops up in my mind..what is max hp on id plate?

 

[sonkilli]

Re: correct power

If i only knew.<br /><br />The hull is originated from US but the ship was rebuilt in Croatian shipyard with very good reputation.<br /><br />There is where the cabin was added to the hull.<br />And i suppose the plate with max HP and type of boat was removed then.<br /><br />So the lack of information makes me searching for a way to calculate correct power for it by myself.<br /><br />It is rather sharp at bow goin flat on the stern, and it looks very firm so im thinking about 130-150HP for it. 225 is a bit too expencive for me at a time. Now i have 70HP but it seams to weak.<br /><br />I can put a picture of it on the web if u are interested in takeing a peek.

 

[LubeDude]

Re: correct power

As big as you can afford. You will never be unhappy with too much HP. But you will be if you do not have enough! You will use less fuel cruising at 3/4 throtle with a large engine than you will at WOT with a smaller engine. Also if you ever get caught in the weather you will like the fact that you can make a run for it if you have to!

 

[cobra 3.0]

Re: correct power

Please post a pic. Maybe someone will recognize it.

 

[ob15]

Re: correct power

Just a thought, but if the HIN is still on the boat, someone may be able to help identify it.

 

[jim dozier]

Re: correct power

Translation into American units<br />23 feet long<br />8 feet wide<br />2600 pounds<br /><br />I have an 17.5 foot boat thats rated for 120hp so you know that 70hp is grossly inadequate for what you are describing. It sounds like you should be looking in the 175-250 hp range. This is a planeing hull (not a displacement hull) isn't it? Money is always an issue of course, people on this forum love to spend other peoples money.

 

[sonkilli]

Re: correct power

Ok... now thats a big response.<br />Thank you all for participating.<br /><br />Yap, its a planing hull or semi-displacement whatever u like better.<br /><br />Picture is on the way.

 

[Forktail]

Re: correct power

You can go here to find out how the US Coast Guard would calculate it...<br /><br />http://squid.law.cornell.edu/cgi-bin/get-cfr.cgi?TITLE=33&PART=183&SECTION=53&TYPE=TEXT <br /><br />

How many HP do i need to run it with optimal fuel consumption to speed ratio.

I'm guessing a 115 to 140 would do you fine. That would be about double what you have now. Remember, the CG formulas are more for weight and safety, not fuel comsumption and speed.

 

[Solittle]

Re: correct power

115-140 would be underpowered for that boat in my opinion. 175 and up would be my choice. I run two 110hp on a 23'er and my performance is adequate but not blazing by any means.

 

[Forktail]

Re: correct power

Hard to say without seeing the boat. 70 HP is doing it now. Plus it's a rebuilt hull with an added cabin afterthought. Flat bottom. It appears expense and fuel consumption are the main concerns for Sonkilli, not speed.

 

[sonkilli]

Re: correct power

Sorry i had some trouble so couldnt reply or post a picture... Ill do it in a couple of days now.<br /><br />As I intent to use my ship in Adriatic sea which is very closed and loaded with isles so the shore is allways near, I am interested primary in running a boat at low cost. So what I need is an motor that will give me around 18-20 knots at 2/3 throttle.

 

[flydangler]

Re: correct power

The Adriatic? You are talking big enough water (at least it's much bigger than my little Narragansett Bay) to consider some degree of safety margin. <br /><br />Maybe your best bet would be to look for a used motor of the same type as you already have to mount side by side. That would double your hp while providing a way back if anything goes wrong with one of them. Just a thought.<br /><br />Good luck!<br /><br />p.s. I believe maximum hp for your hull is about four times the 70hp you're using now.

 

[wtrojan]

Re: correct power

Sonkilli, I have a planning boat roughly similar to yours, longer (25 feet, about 7.6m), about as wide as yours and I think that my is also heavier. No cabin, but a beefy T-top. With a 140HP it runs perfect for me, getting more than you say you expect (2/3 throttle and 20 knots). With about 200-300rpm above 2/3 throttle I get about 25 knots easily. So my guess is that although the bigger the better, starting with 130-140 you would be done, considering also the $$$ factor.<br />However, you have to be aware of other important factors that will influence the boat performance. First, the motor about this big is about 400lb (200kg) more of weight. Another thing is your fuel capacity. Cruising with a big 200HP two stroke you will use about 12-13 gallons per hour (roughly 50 liters). Does your boat have adequate capacity? Do you consider fuel’s weight? I guess you could easily get to several hundred kilograms of fuel with full tanks. Add crew and stuff.<br />Another things that will affect the performance are the engine on the transom height (X dimension) and propeller size (mostly pitch). Be prepared for some trial and error on these two. Good luck.

 

[sonkilli]

Re: correct power

Thanks all and please accept my apologize for not reponding so long to this. I had some troubles. I took in consideration all you people wrote in respond to my question and also did some investigation myself and finally i mounted single 4 stroke 150 HP Yamaha on board. It runs 24 knots at 3200 rpm out of 5500 max and im very pleased with fuel consumption so far. If anyone is interested in any information please post and I'll report back as soon as I can. Ah... my boat is allmost same as Parker 2120 Sport Cabin.<br />By...