DT-85 , how many rpms @ WOT ???

[bowhuntrrl]

I'm having trouble getting on plane so I just dropped down from a 13 1/4x19 prop to a 13 1/4x17 pitch. I just installed a new tach but haven't had the boat out yet. Does anyone know what the max rpms should be at WOT ??? I just want to make sure that I don't over rev the boat. The motor is a 1987. Depending on weather, I'm hoping to get the boat out on Tuesday. Thanks.

 

[deejaycee_2000]

Re: DT-85 , how many rpms @ WOT ???

5500 - 6000 rpm

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

What type boat hull is it , length and weight? Where is the cavittation plate in relation to the keel at the lowest point. Most boats have the motor cav plate at or +- an inch or so from that point.
Example: I have an 18ft. Maverick boat (similar to a bass hull)with a factory weight of 1100# plus weight of gas and accessories and run a DT140 with a SS 13-1/4" dia X 19" pitch cupped prop with WOT at 5780 rpm. For skiing purposes or with additional people loads only I install a SS 13-1/4" dia X 17" cupped pitch prop. Using the 17" pitch prop without skiing/persons it will tach rpms around 6000 in which I throttle it at 5700. I very seldon use the 17" but I have it on hand when I need it.
With my previous DT115 on the same boat I used the same 17" pitch prop and it ran about 5700 rpms at WOT. The total mph was only about 5 less than that of the DT140 now. Both had great hole shots, with the 140 better.

Point is the boat you may have with the 85hp may need a smaller pitch prop than 17". Having that tach will give you a better understanding were you're rpms are at and make it more selective in determining prop size. Note a cupped prop is a good asset to have and should be taken into account for the final prop selection.

There are some previous post here for determing prop sizes based on boat/rpms/etc.
Good luck

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

Thanks guys. I have a good understanding of prop pitch/application. I just wasn't sure of what rpm range I was looking for. When I last had the boat out, it didn't sound like it was revving very high and like I said, the hole shot was bad, taking a long time to plane. I realize that I may have to adjust down from where I am to maybe a 15. The boat is a SportCraft 16' CC. The cavitation plate is about 1" below the hull. I normally like to have them just even and may eventually move the engine up 1 hole on the mounts to achieve that. The previous owner told me that the boat only does about 25 mph, which I feel is slow for the combination. I've spoken with others that have the same boat and with a 70 hp Evinrude, one of them claims 32 mph. When I take it out I will also check the speed with a GPS to see just where I'm at.

 

[deejaycee_2000]

Re: DT-85 , how many rpms @ WOT ???

keep with the 17 p prop, just adjust your motor so the cavitation plate is in line with bottom of boat ......

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

Had the boat out today and the bad news is that it will only tach 5000 rpm @34 mph top speed. This is a new tach installation , but the tachometer seems to be working fine, showing the idle around 700 rpm. I'm really having a hard time believing that I need to run a 15 pitch prop, and even with that, I'll probably still be under the power band, estimating that the 15P will run at 5300-5400 rpm. Also, will I lose speed with the 15P ???

Even if I bring the motor up 1 hole on the bracket, I really don't think that it will give me any more rpm, will it ???? I suspect that might only give me more speed, maybe 1-2 mph.

The motor starts good, revs good when I mash the throttle, and seems to generally run good so I don't think it's a motor problem, although I'm not ruling out that possibility. The compression runs between 125-132 between all 3 cylinders. Could it be a timing issue ????


I'm going over later to take a few pics of the boat and maybe post one here so you can look at the hull design. Any thoughts will be appreciated.

 

[pootnic]

Re: DT-85 , how many rpms @ WOT ???

is it possible your WOT isnt actually WOT...i mean is it possble your not all the way opened up?/old cables something not ajusted right in the carb. not allowing it to go WOT...im know mechanic but maybe with no prop on your still not getting the right rpm's....just a thought

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

is it possible your WOT isnt actually WOT...i mean is it possble your not all the way opened up?/old cables something not ajusted right in the carb. not allowing it to go WOT...im know mechanic but maybe with no prop on your still not getting the right rpm's....just a thought

Nope !!! After the carb overhauls, I checked to make sure the butterfly's were opening up all the way.

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

more pitch less rpms. less pitch more rpms.
Unless that prop hub is weak and causing the prop to slip and lose some rpms using that prop is about all you will get out of it. Raising the motor a notch may or may not have the effect on top end. Raising the motor you may run into the prop bade slippage in the water, not having enough water over the blades. Also on turns if the motor is too high on the transom the prop will lose its bite.

Is that 5000 rpm/34 mph with the motor trimmed up to tweek the highest rpm/mph readings? And was the boat full of gas and all weighted accessories?
Usually when you change from a 13X17 prop down the will be a larger diameter...13-1/2X15.The larger dia. makes for more RPMS i.e. more speed in most cases. Cupping the prop reduces the rpms but it gives the prop a better bite. As an old rule of thumb each "pitch" reduction, adds 200 rpms .... of the same diameter prop.

I was looking at an old Suzuki brochure I had and it has an 86-87 DT85 RPM range at 5000-5600.
If that is true then you're very close to meeting the RPM range design.

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

Hopalong,

Thanks for getting back.. Actually, I have some less than ideal conditions with the boat. When I bought it 2 months ago, it had been sitting for over 4 years. The fuel tank is almost full and I haven't figured out a way to get rid of it yet, so I'm carrying all that extra weight. I've been running off a new 6 gallon plastic tank. So there's a bunch of weight there although I'm not sure of how many gallons the tank is, it's at least 30 and may be as much as 50-60. There is also some wet foam in the boat which will be a springtime project for me to try and dig it all out.

Secondly: The motor is not at the ideal height. The cavitation plate is about an inch to 1 1/2" below the keel at the transom. When running the boat today, I actually was trimmed up all the way (actually started to tilt once), and still couldn't get it to cavitate on sharp turns. If I have time and help, I'll try to raise the motor up 1 set of holes in the bracket tomorrow. My thought on this is that I may pick up maybe 1-2 mph. My real question is if I will pick up any rpm. I'm hoping that raising it up will let me trim the boat higher out of the water since even trimmed up all the way, the nose doesn't seem very high. My thought is that the extra friction from the water may be holding it back and not letting it achieve max rpm or enough trim to get the hull out of the water.

The prop is a Michigan wheel Vortex with the hub system. They are not cupped and usually run 300-400 rpm higher than a cupped prop, according to the outfit that I bought it from when I called them today. The prop is brand new as is the hub kit. These hubs can't slip, only break.

It was on this forum that I was told that the WOT power band is 5500-6000 rpm. I can live with the speed since It's not often that you can run at 34 mph on the ocean with a 16 footer, I just want to get it set a bit more optimum as far as the power band goes. I have the option of sending it back for a 13 3/4 x 15P. That should help about 300 rpm, but what do you think it will do to the speed ????

The really sad thing is that this boat/motor combination has been like this since new. The original owner told me the boat would only do 22-25 mph. It looks like it was set up by morons !!!!

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

I'm far from an expert on any marine associated boating issues. But I have had a lot of practical experience and findings over the past 30 years with boats I've own and operated and helping others with their boats.

To answer some of your questions and please know I'm only giving opinions and hope I'm not going "overboard" with my direct manner in which I speak.

quote-it had been sitting for over 4 years. The fuel tank is almost full and I haven't figured out a way to get rid of it yet, so I'm carrying all that extra weight. Note water weighs about 8# per gallon, so a 30 gallon gas tank when full may weigh about 240 #. Boat manuf. size and install built-in gas tanks safely and so as you will have adequate supply of gas to the outboard motor for many hours of usage. The boat is designed for this weight and the transfer of the weighted gas when the boat is under way. If you remove this weight/tank and only use "6" gallons of gas in another area of the boat that quanity of gas will reduce your travel time significantly. And if the 6-gallon gas tank is not USGC approved for above deck and or not installed per instructions you may have to suffer some consequences later. An 85 hp outboard uses a lot of gas per hour even cruising at normal speeds.
To remove the gas from the existing tank you can use a manual primer bulb/hose kit from Pep Boys or elsewhere. Works sorta like the fuel primer bulb and comes with clear neoprene hoses. Note the tank probably has vertical placed baffles that keep the gas from moving about when the boat is under acceleration etc. And sometimes it is almost immpossible to remove all the gas since the filler opening is located maybe in the area of one of many baffles. Once you remove the gas to a safe container you must take it to a registered source to dispose of it. And if you do not to intend to use the built-in tank it will still have "gas fumes" within that is hard to rid of unless you use cleaners etc. The dangers of removing the gas tank from a boat is very unsafe and should be taken with the upmost care. All it would take is a spark to explode the fumes!!! All fumes must be expelled before any attempt is make to remove a tank!!!
You would be wise to try and reuse the tank if it does not leak and fill it to gallon limits of your choice. The small portable 6 gallon tank usually is not enough for a day on the water for an 85 hp outboard.
Secondly: The motor is not at the ideal height Yes tweeking the motor height and or trim usually help in higher rpms. Is the built-in gas tank forward of center of the boat? Do not wax the bottom of the hull...causes drag...just besure it is smooth.
I have the option of sending it back for a 13 3/4 x 15P. That should help about 300 rpm, but what do you think it will do to the speed ???? I would suggest to tweek all you can get with the motor and boat using the prop you have for the highest rpms and speed. Then decide were you are with the top end speed and rpms. If it is not were you need it to be THEN change to another prop size. Remember cupping the prop may loose some rpms but it usually is a significant gain on hole shots, turning, and bite. I use a Bob's Machine plate device mounted on top of the cav plate to get a great hole shot in conjunction with a proper pitched cupped prop to move the boat forward to obtain WOT with the boat fuel tank full and all accessories and myself on board.

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

Here is my rig........

http://www.flickr.com/photos/12505056@N06/1302828381/in/set-72157601816534254?edited=1

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

You must have misunderstood me. I know how to physically get it out, I just don't have a way to dispose of it all !!! I have a marine electric fuel pump that I used to "polish" the fuel in the last SportCraft that I owned. I pumped it from the tank, through a filter, and back into the tank for 10 hours to remove some contamination. I'll use the same pump to empty out this one.It's just a real problem getting rid of anywhere from 30-60 gallons of 4 year old gasoline. The tank I'm currently using is a Tempo, I assume it's CG approved since it's sold for small craft use. The current tank runs from about 2' aft of the CC and runs all the way up to the front seat of the console. The cover is over 6' long. I have plans in the spring to pull the cover and possibly the tank, even though it is foamed in. I have some wet flotation foam that I need to remove. The previous owner left it uncovered for 2 weeks after I bought it, but before I picked it up. The plug was in and the bilge filled up. The mice had been in there and chewed about 2 bushels of foam into popcorn sized pieces. So when the water rose, the foam got soaked. I got the boat for such a good price, I really can't complain, but it is an ongoing project.

When I finally empty the tank, I will start using it, particularly since the Suzuki has a good oil injection system.

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

Sounds like you have another project other than the outboard prop issues. I assumed that you may not have known what one could be getting into with the gas tank. You would think that the county you live in would have a county disposal site for bad gas and other chemicals. Most all counties in the US are/should provide this for the home owner other than just trash and garbage pick up. Here in my county one only has to "take" the chemical, paint, etc. to the site.
When you get ready to install foam into the boat check into the forum for the new foam products that are available. They are a lot better than the old stuff.
Good luck on the final remedy for your prop issues. At least you are running in the 30 mph range. Hitting the 40mph @ 5600 rpm with some hull designs and other related issues sometimes just dont happen. That cav stabilizer plate shown in the post before helps in trimming the bow up faster in the hole shot and also once on the can increase top end speed by trimming the bow up when the conditions fo some boat/OB combinations wont. I think there only $45 or so?
Here is the website for Bobs Machine. Checkout some of the items they carry other than the stabilizer. They are located local here in the west coast of Florida.
http://www.bobsmachine.com/

 

[bowhuntrrl]

Re: DT-85 , how many rpms @ WOT ???

[FONT=Verdana,Arial,Helvetica][FONT=Verdana,Arial,Helvetica]Here's an update: I got the 13 3/4 x 15P prop. I had the chance this evening to take it to the lake. There was just a slight breeze putting some light ripples on the water, conditions were very nice. From the moment that I mashed the throttle down, I knew we were in business. I had forgotten to trim the motor down and the boat stood straight up !!!! I trimmed it and it popped right on to plane. I stopped it, trimmed it down and tried the hole shot again. It jumped right on to plane with no fuss at all. The top speed was still 34 mph, only this time @ 5500 rpm. I can't believe that dropping only 1 pitch raised the revs over 500 rpm, but it did. I tried to find a good cruising speed and came up with 26 mph @4200 rpm. That should be a good compromise speed to save gas. Overall, I'm pretty happy with this Michigan Wheel Vortex prop. The only upgrade that I might do in the future would be to a 15P stainless which might give a few more mph and a few hundred less rpm.

Our next experiment is to raise the engine up 1 hole since the cav plate is about 1" below the bottom of the hull. After that, I will install the Bob's machine torque plate to see if that changes anything.[/FONT][/FONT]

 

[hopalong]

Re: DT-85 , how many rpms @ WOT ???

You're getting there. A SS prop will make a difference plus cupping one adds to a better bite. The Bob's plate works great....