Car Motor For Inboard

[kobus]

Hi all,some help please i want to install a 1400cc twincam motor 75kw in a rubberduck,this motor pushes about 80hp @ 5000rpm can i use a direct driveshaft through the hull.keep it plain and simple no gearboxes what pitch prop should i use to get the moast out of my motor?

[Jango]

Depends on size of the Boat. Also,need total weight with gear and passengers to determine Prop.

[kobus]

Hi Jango,thx for your repl.the boat weigh in at 400kg plus say 4 passengers including skipper so total weight 650kg boat size 5m x 1.2 wide rubberduck i want to run a direct drive through hull at 4000 -4800 rpm or maybe a gearbox like the outboard 100hp yamaha at 1-2.15 ratio
thanx kobus

[Jango]

For inboard with straight shaft thru the bottom -Not sure what the bottom looks like on your Boat. Figuring 700kg total, Prop dia (3 blade) should be 9 1/2 to 10 in. Pitch should be 12 for Flat bottom, max speed abt 44 mph. If Bottom has alot of Vee, 10 pitch and 34 mph. I would start with the 10 pitch and go higher if it overrevs

[kobus]

Thanx jango,the bottom is a semi v shape at the stern.ill try the 10 in.then
one more question,a friend suggests that i use a radiator in the front of the stand up console to keep it a closed circut for cooling,alternatively i can use river water for it is fresh and around 18deg.celcius.any sugestions?

[Jango]

The radiator idea is OK. Remember though, Air must be able to flow thru. You can't have anything behind the radiator preventing airflow. With a radiator, you will NOT need to marinize the cooling system - water pump, gaskets, BUT WILL HAVE "hot exhaust pipes" which could be a problem.

[kobus]

thanx for the tip,ill make sure that air can go in and out of the centre console,one more problem, cooling the exhaust manifold and the exhaust pipes????the engine is a standard 1400CC car motor can i use heat resistant material or should i go for water cooling,and to water cool a manifold how do i go about that???

[Jango]

In order to water cool the Exhaust, you will need to Install a Marine Exhaust manifold, which has water jackets built-in around the exhaust chambers and exits the water into or near a water resistant exhaust pipe into the river. This system will require a water pump to get water from the river/lake.
OR if the motor is located such, that it will be safe to just wrap the auto type exhaust with a heat/fire proof material and not water cool it. Exhaust system should have plenty of space around it so as not to cause a fire, even with fire resistant wrappings

[Sceptre]

would you like to share pictures or anymore info about ur project ? because i have a 16ft chrysler trihull carbonfiber boat with 85hp outboard that can carry upto 5 people I work in a mechanical shop so i was thinking of mounting a 1.5 honda civic engine onboard and runing a streight pipe or through gear boxes any sugestions or advice ?

[Boston]

you will loose some efficiency at 5000 rpm
at that speed your prop is going to be small
smaller props loose more to tip effects and slip
the most efficient thing to do is have a large prop turning slowly
if your looking to burn through the water then its still the same concepts involved
larger prop turning slower is always your most efficient plan
at 5000 rpm your going to be getting terrible losses at the prop and in terms of life span of the engine

the best auto motor for this application is always a diesel
huge torque at lower RPM and they are build to handle the demands of marine use much better than the gas option

[Sceptre]

you will loose some efficiency at 5000 rpm
at that speed your prop is going to be small
smaller props loose more to tip effects and slip
the most efficient thing to do is have a large prop turning slowly
if your looking to burn through the water then its still the same concepts involved
larger prop turning slower is always your most efficient plan
at 5000 rpm your going to be getting terrible losses at the prop and in terms of life span of the engine

the best auto motor for this application is always a diesel
huge torque at lower RPM and they are build to handle the demands of marine use much better than the gas option

so you recomend geting a biger prop and a diesel i dont understand how will i loose power if i jus gotta spin one prop the engine is ment to be hauling upto 2 tons

[Boston]

definitely
most of the efficiency losses will be due to prop dynamics
essentially a faster spinning prop has more losses than a slower prop
the questions of diesel or gas is easily answered by considering that in a marine application engines are subject to far more constant loading than in a passenger car application
think of it like the same reason most cars run on gas and most trucks run on diesel
when under constant load at a variety of speeds the diesel wins out every time

prop dynamics are a complex issue but basically slower means less cavitation at non optimal speeds reduced friction lower tip losses and less slip
look up "propcalc" and load the application
then play with lower rpm and torque and see what props it throws out to you
you will notice that when you increase RPM beyond a certain point you decrease efficiency
5000 RPM is way beyond the average prop and I think
although I may be wrong on this beyond even racing applications
most engines want to run in the 3000 range for peak HP and at that most are stepped down 2:1

[Sceptre]

ohh aite man thanks a lot i understand now I apreciate your reply so basicaly i get the feeling that car engine is no good for this

[Boston]

Hi all,some help please i want to install a 1400cc twincam motor 75kw in a rubberduck,this motor pushes about 80hp @ 5000rpm can i use a direct driveshaft through the hull.keep it plain and simple no gearboxes what pitch prop should i use to get the moast out of my motor?

well it does not work for this plan specifically

if you stepped down the rpm to say the 1000 range and your weight distribution and total weight were relatively the same as for the engine your replacing then it would work other than gas will get a lot less millage than diesel specially in marine use.

lots of folks use gas engines in various boats around here but they have gear boxes and couldn't care less about millage. They are also more typically into water skiing than anything else.

[technik]

Kobus is this motor honda nissan toyo or mitsu,is it efi or carb or cpi?carb and cpi kind of look similar,cpi will have two large wires going into the top of the throttle body under the air cleaner.if its cpi honda and has what I call a vacuum computer(fire wall mounted black box with 20+ vacuum lines running in and out)run don't Walk to a new option.if your mechanically inclined I can walk you through changing the cam timing to bring the peak torque rpm as low as possible depending on the engine management system type.

[baeckmo]

Hi all,some help please i want to install a 1400cc twincam motor 75kw in a rubberduck,this motor pushes about 80hp @ 5000rpm can i use a direct driveshaft through the hull.keep it plain and simple no gearboxes what pitch prop should i use to get the moast out of my motor?

NO YOU CAN'T GO DIRECT DRIVE! Boston has given the basic reason; you might say that the prop needs a certain "swept area" (we call it propeller disc area) per kW, depending on the boat speed.

Without going into calculation details, you can check suitable propeller diameters on outboards of similar power as your engine. Now, first you say it is 75 kW, that is 102 hp. Then you say it will produce some 80 hp at 5000 rpm; there is 20 % difference! Anyway, you will find that outboards this size will use propeller diameters between 13.25" and 14". This is the statistical outcome from the most common applications of this engine size, giving reasonable performance and efficiency.

This means that for your engine, you have to use a step-down gear ratio of between 1.8:1 and 2.0:1 in order to match engine power to this propeller dia. A direct drive 5000 rpm would just stir a lot of water and produce no thrust; the maximum prop dia would be ~9". The only way to go direct drive would be through a waterjet, but even there, 5000 rpm is on the edge, depending on boat characteristics.

[Jango]

A 650 kg total weight, with 80 HP @ 4000 rpm, 1:1 gear. Propcalc shows a 10 1/2 x 11 P. resulting in 35+ mph @ 13% slip.
Shouldn't be a problem at all - Many years ago, I had similar situation, 100 + HP @ 4200 rpm, 850 kg, 11 x 13 prop, 46 mph.

PropCalc indicates 1.5 or 2.0 : 1 gearing is not recommended for this application [ 4000 - 5000 rpm engine speed]

[Jango]

D.A.R. [ DIA - AREA - RATIO ] Has little to do with power. Most 3 bladed props have a D.A.R. of 55% which means the total area of the blades is equal to 55% of the area of the circle created by the revolving prop. Outboard Props are larger in order to obtain the required blade area due to the Large Hub.

My current 2250# inboard runabout, with 450 HP uses a standard, no cup, 12 x 15 prop - 55% DAR, Turns up to 6800 rpm with LESS than 7% Slip, 87 mph gps. This "tiny" prop is able to instantly put the ENTIRE BOAT Airborne from idle speed. Hardly "just stirring a lot of water and producing no thrust"