Am I spinning my prop enough rpms?

[tmt]

Boat 1997 Wellcraft 190CCF
Weight around 2000lbs

Motor 1997 Johnson 115 spl, set at one bolt hole up

Prop is a Power Tech rxb4r15pom140 that I bought new a year ago has some where from sand bars. The diam is 13.25" pitch is 15 and is SS four blade.

With 1/2 tank of gas, 2 people, at wot if I continue to trimed up the prop blows out at 5000rpm. If I stop trimming before the prop blows out I don't get any more rpms or mph, 34 with gps.

With the boat loaded 4/4 on gas, coolers, ice. wot is 4800rpm at 31mph.

I normally run the motor from 3800rpm to 4200rpm at about 24 to 26 mph.

My concern is am I harming this motor?
prop link http://www.ptprop.com/component/pag...acturer_id,0/option,com_virtuemart/Itemid,46/

Or will setting the motor down to the transom help?
Or should I have this prop repitched to a 14" or 13" to bring the rpms up to 5500?

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Tmt, you are running within the manufacturers recommended RPM range,

With the boat loaded 4/4 on gas, coolers, ice. wot is 4800rpm at 31mph

But you are running at the bottom end of the range for your motor, and it would run much more efficiently if you were at the top end of the spectrum, and with increased engine life, in my opinion.

I normally run the motor from 3800rpm to 4200rpm at about 24 to 26 mph.

You also are running very high RPM at cruise, considering that the max RPM are only 4,800. This means you are cruising in the top 13% of your maximum RPM and I find that to be a little hard on the engine and probably not very good gas mileage since the boat is under a load already, but that may just be a matter of differences in how people use a boat.



H

 

[tmt]

Re: Am I spinning my prop enough rpms?

Hwsii wrote
it would run much more efficiently if you were at the top end of the spectrum, and with increased engine life, in my opinion.

If I reprop to reach the top end of the spectrum (5500rpm) and cruise at 4500rpm to run 25 to 28mph will my fuel consumption be less or the same?
What type of prop would you suggest to reach this?
Does less of a load on the motor help with all around performance, fuel/motor life?

 

[steelespike]

Re: Am I spinning my prop enough rpms?

Boat is 2,350 lbs. and apparently at max hp 115.
It seems to me that you should do better with a 15" pitch.If you have prop wear from sand you have probably lost some cup, maybe even pitch and maybe even diameter under severe conditions. It appears if you can keep it hooked up you could get more speed and rpm.You could start by having the prop reworked back to original specs and check the results.May lower your cruising rpm.The idea is to get as much speed as possible while staying within the rpm range.If the motor can reach its max rated rpm at wot(wide open throttle)
yet cruise higher speed with less throttle the motor will be more comfortable
and economical.
Where is the antivent(cavitation) plate (just above the prop)in relation to the bottom of the boat?

 

[tmt]

Re: Am I spinning my prop enough rpms?

The boat has a step up on the bottom where the motor is mounted. Sets the motor back about 18" from main bottom. It looks like the cav. plate is higher than main bottom.

 

[steelespike]

Re: Am I spinning my prop enough rpms?

Sounds about right.You might check at speed to see if the plate is clearly visable.

 

[reelnative]

Re: Am I spinning my prop enough rpms?

you would do a lot better if you changed to a 3 blade prop, 2 cycle motors are not ment to run with laods on them at low rpms thats were damage happens, thats why you want a prop that will spin up to the higher side of your rec rpm range, a 115 is 5500 I think, a good prop setr up for that boat and motor would be a 13.25- 17 sst 3 blade

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Tmt, I find your gear ratio to be 2:1 but I am not sure what your maximum RPM should be, because they made 2 motors that year with 2 different RPM ratings. One motor had a maximum of 5,500 RPM and the other was rated for 6,000 RPM, please verify which one you have and gear ratio.
And Spike is very right about losing the cup and potentially the pitch, as always.

1.If I reprop to reach the top end of the spectrum (5500rpm) and cruise at 4500rpm to run 25 to 28mph will my fuel consumption be less or the same?
2.What type of prop would you suggest to reach this?
3.Does less of a load on the motor help with all around performance, fuel/motor life?

1.It will be less.
2.I need more information.
3.Absolutely

A.The boat has a step up on the bottom where the motor is mounted. Sets the motor back about 18" from main bottom. It looks like the cav. plate is higher than main bottom.

A.can you get me about 4 pictures of this and measure how high the anti ventilation plate is above both bottoms, with the anti ventilation plate level with the bottom.

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

Motor Transom Height

When the boat is running in the top 20% of your speed you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.

Correct Anti Ventilation Height

Let's look at the setup of the motor height before we talk about changing props, as that is the cheapest and best way to at least start.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057



H

 

[Marty Niner]

Re: Am I spinning my prop enough rpms?

Thanks for the info guys, I was having the same problem with my boat. I think I need to drop the motor a hole or two. Whenever I take a turn at speed I get ventilation, and I have to run with the motor trimmed right down.

 

[steelespike]

Re: Am I spinning my prop enough rpms?

Its better to post your on topic if this goes any further it could get confusing.
Yes the least costly way to stop venting is to lower the motor.
It is possible that a better prop might let you run that height or even higher.
Do you know where your plate is in relation to the bottom?

 

[tmt]

Re: Am I spinning my prop enough rpms?

hasiii asked, can you get me about 4 pictures of this and measure how high the anti ventilation plate is above both bottoms, with the anti ventilation plate level with the bottom.

gear ratio is 2:1
max rpm 5500-115hp
boat 2300 lbs-19ft
motor 320 lbs
gear and people 500 lbs
fuel 50gl
live well 20gl
batteries 2

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Tmt, I haven't seen your particular hull before, but you mention that the motor is setback 18" from the 1st bottom and the pictures you show don't appear to show that much distance between the transom and the first bottom. That will make a very large difference in how high the motor should be mounted on the transom. From the pictures the design of the stern of the boat and the distance between the first bottom and the second bottom doesn't look like 18".

And I do want to commend you on taking excellent pictures.

It shows that the anti vent plate is 1 1/2" higher than the first bottom.

The boat has a step up on the bottom where the motor is mounted. Sets the motor back about 18" from main bottom. It looks like the cav. plate is higher than main bottom.

I also show your prop slip to be 4%, which is exceedingly low, in my opinion, and thus you are actually wasting power from blade friction that could be put to much better use in forward propulsion, because this friction is also stopping your motor from reaching its maximum RPM.




H

 

[tmt]

Re: Am I spinning my prop enough rpms?

From the lower keel (very bottom of boat) to the prop is 27".

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

How many inches is it from the back of the lower bottom to the transom.



H

 

[tmt]

Re: Am I spinning my prop enough rpms?

27"
sorry it is 7"

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Great, now I have something to work with, I will get back with you in the morning.



H

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Tmt, you have two different problems going on.

1. You are running a hydrafoil on your anti vent plate that is at least 2 1/2" below your transom. This is creating drag and eating RPM from your motor.

2. Your anti ventilation plate is also in the same place and that means your lower gear housing is also eating up RPM and speed as well.

Walleyehed and dhadley recommend running motors like yours at about 5,800 RPM at WOT, to keep the motor from coking up with the new fuels we have now. And they know what they are talking about, so this is actually what would be best, but I don't think we will reach that with my proposal, but we will be much closer.

Motor Height

If you will look at the above picture you can see that the anti ventilation plate is about 2 1/2" below the rear step on the boat, in otherwise I mean the transom. And as you said, there is a 7" setback from the lower hull to the transom.

In my opinion, you need to take the hydrafoil off and then raise the engine up two notches on the transom, this will increase RPM and speed. I also believe that your motor holes possibly should have been placed higher when they were installed. I do believe that we could raise the motor all the way to the top of the transom, but I am not positive, because of the angle between the 1st step and the 2nd step, so let's just do two the first time.

You need to then take the boat out and run it and get us some new numbers, I notice that this particular prop doesn't have a lot of rake or cup built into it, so we may have a slight problem with ventilation, but I do NOT believe that will be the case because we have four blades. When you get back then we will go over the new results and map out the rest of our strategy.

But be sure to watch the water coming out of the pee hole on the side of the motor when you are at and above cruising speed, I am just making sure that we don't have a problem with water circulation in the motor.

And like Spike said you certainly seem to have been on sandbars or mud bars a lot from looking at the bottom of your lower unit. Remember, that doing this will also eat up water impellers very quickly because of the extra contaminants, that is why I mentioned the pee hole.



H

 

[tmt]

Re: Am I spinning my prop enough rpms?

Ok, I will get back to you with new numbers. I did take the hydrafoils off before and the boats bow slapped up and down uncontrolably.
I wan't to thank you so far with this.

tmt

 

[hwsiii]

Re: Am I spinning my prop enough rpms?

Tmt, that is not a good sign, it theoretically means that the prop you have does not have enough rake to hold the bow up. Some of the time the faster you go can sometimes make it stop and sometimes if you move people to the front of the boat it will slow or stop the porpoising, because the more RPM the prop turns the more bow lift it is capable of producing. But I doubt that will happen in this case, so bring the hydrafoil with you and if you absolutely can't reach maximum speed, then attach the hydrafoil again.

But, it is very important to have at least 4 pictures showing how high the anti ventilation plate is running in the water, so be sure and have another person with you, it should absolutely be on top of the water and not underneath it or we will still have problems with the extra drag,

I know all of this is a PAIN, but to get your boat running the best we can I do not know of any other way.

And you are very welcome Tmt.



H

 

[tmt]

Re: Am I spinning my prop enough rpms?

Got new numbers, the picture are at WOT trimed up just before porpoising or prop blow out.

4900rpm

35mph

motor is up 3 on transom 1 more left

no hydrofoil