OMC 305.... top speed questions. Who has one?

fabrimacator21

Petty Officer 1st Class
Joined
Jun 28, 2009
Messages
286
Boat weighs roughly 3500 lbs and with trim gets out of the hole great with the 17 pitch prop I have on it right now. I'm considering a 19 but want to know if a 21 pitch would be too much for the 200hp 305. Anyone running a 21p prop with a 305? What is your top speed and rpm?
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

Fabri, we will try to help you as much as we can, but there is certain information that is needed in order to help you. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information, and most other people who are trying to help you will need the same information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the exact prop you need for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor
8. Manufacturer?s recommended WOT range
9. Antiventilation Plate height above keel of boat if it is an outboard in inches
10. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from each prop if more than one prop
The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H
 

fabrimacator21

Petty Officer 1st Class
Joined
Jun 28, 2009
Messages
286
Re: OMC 305.... top speed questions. Who has one?

Thanks for the great reply!

I will try to give you as much as I can but there are a few grey areas...

1. 1978 galaxy Trihull (in my avatar)
I have searched all over and can't find the model of the boat.

2.Length is 18'..... weight is 3500lbs
3. 3 to 4 people with a 22 gallon full tank
4. Mainly fishing and cruising but I will be pulling skiers, tubers, and wakeboarders occaisionally.

5. No "V".... it's a trihull
6. Motor is a 1978 OMC 305
7. Rated at 200 HP...I/O gear ratio is 1.83:1
8. Don't know the recomended WOT rpm as I don't know the model but I would assume it's the same as any other 305... I'd say 4500-4800
9. N/A... it is an I/O
10. Omc 1978 aluminum 17p prop
11. only one prop
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

Fabri, I need the WOT RPM and speed you are turning right now, and is that outdrive the 800 model, I also show the WOT RPM at 4,000 to 4,400. Are these right.

H
 

fabrimacator21

Petty Officer 1st Class
Joined
Jun 28, 2009
Messages
286
Re: OMC 305.... top speed questions. Who has one?

Fabri, I need the WOT RPM and speed you are turning right now, and is that outdrive the 800 model, I also show the WOT RPM at 4,000 to 4,400. Are these right.

H

See that is where I'm sort of stuck... my tach doesn't work and my speedo needs a new transducer. So I really can't tell you either for sure right now. Yes the outdrive is the 800 model. With the 17p it sounds like its close to 5000 rpm... maybe 4600-5k. And top speed I would guess is 45. I'll try to nail down some numbers next time I have it out but right now thats all I can offer. Thanks.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,099
Re: OMC 305.... top speed questions. Who has one?

my tach doesn't work

Ayuh,... With out rpms at WOT,... Your questions are Unanswerable...
 

proshadetree

Lieutenant Commander
Joined
Jul 19, 2008
Messages
1,887
Re: OMC 305.... top speed questions. Who has one?

I have a 305 cobra in a 1952 cuddy it turns 5200 and the speedo shows 45 to 50 on glass smooth water with a 14 1/2 19 prop.I know this isnt your boat and isnt gps speed but its what mine acts like.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

Fabri, I have to know the Factory recommended WOT RPM range. I need that information. I looked it up but I show 4,000 to 4,400 and then shadetree says his turns at 5,200 and that is too much of a difference for me to calculate.

H
 

fabrimacator21

Petty Officer 1st Class
Joined
Jun 28, 2009
Messages
286
Re: OMC 305.... top speed questions. Who has one?

Ok.

It's gonna be a week or so before I can get speed or WOT rpm for sure...

But My dad owned this motor in his boat for 10 years and he's telling me it's at 5k. Can you just put 5k and 45 mph in for now?
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

How many people were in the boat when you were at that speed.

H
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

Fab, if you are turning 4,800 RPM on a 17" prop with a 1.83:1 gear ratio the maximum theoretical speed with 10% prop slip is 38 MPH. I ran my prop picker and it says you should be using a 19" pitch prop. I then took this 4,800 RPM with a 17" prop and changed it to a 19" prop and I show a drop in RPM of 400 to 4,400 RPM. Here are the numbers.

Prop Picker

fabrimacatorPropPicker.jpg



Prop Change

fabrimacatorPropChange2.jpg



H
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: OMC 305.... top speed questions. Who has one?

Fab, I have searched and searched, but if we can get the recommended WOT RPM and gear ratio right I can get the speeds right.

H
 

fabrimacator21

Petty Officer 1st Class
Joined
Jun 28, 2009
Messages
286
Re: OMC 305.... top speed questions. Who has one?

I have a 305 cobra in a 1952 cuddy it turns 5200 and the speedo shows 45 to 50 on glass smooth water with a 14 1/2 19 prop.I know this isnt your boat and isnt gps speed but its what mine acts like.

What is your hp rating and gear ratio?
 

proshadetree

Lieutenant Commander
Joined
Jul 19, 2008
Messages
1,887
Re: OMC 305.... top speed questions. Who has one?

Cant tell you.Its a 305 out of a 96 truck and the outdrive is off a 4.3 v6.Its a mix and match that was just to get me in the water that I now just love.
 
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