88 doral Tera rpm to high?

Sbrent

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I just bought an 88 doral Tera cuddy cabin 22'. It has a 4.3 and an alpha one. It takes 3000 rpm to barly reach plan but works good at 3200. When trim is adj 3200 rpm the boat is going 32kph. I think it is revving to high I only what to go cruzing around don't really care about speed I want to get the rpm down around 2500 or less and stay on plane is this possible? Right now I have an al. Prop with 14.5 x 19 pitch!

Ps this is my first time posting on this site any info would be appreciated I also would like to have a manual on the boat is anyone knows where to get one!!!
 

steelespike

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We need to know your lightly loaded wide open throttle rpm and gps speed. Your motor needs to be able to rev freely within its wot rpm rating.
Then your can run at what ever rpm you prefer.
What is the hp of your 4.3?You don't want to pick an arbitrary number to cruise at. Through testing you need to pick the rpm that goes the fastest at the
best fuel use/throttle setting.
 
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Sbrent

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Ok well I don't know my wot rpm because my boat is an 88 and I don't care to use wot. The 4.3 has about 205 hp. As I said it's an 88 and I don't want to be running at 3/4 throttle all the time for longevity of the engine and fuel economy purposes. Therefore I would like to be able to keep my rpm a little lower. The wot rpm of the 4.3 is about 4300-4500 depending on the year ( info I found online)
 

Sea Rider

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Sbrent,

Steelespike has given you great boating advice, follow it if wanting best engine performance throughout the whole throttle range provided that engine runs inside safe wot rpm range, only possible to check with tach. Can't tell if revving too high unless you have a musician tunned ear, if not, a tach will tell you so. Outboards having direct transmissions usually revs higher than cars specially at wot, but that's normal, were designed to work that way.

Happy Boating
 

Sbrent

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Yes I agree it is great advice thanks. But your saying best economy through the throttle range, if that's the case than I only have a 1000 rpm range! Is there anyone out there who has about an 4000 pound boat with a 4.3??? What is your throttle range??
 

steelespike

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The wot rpm range is 4400 to 4800 rpm.
I'll try one more time. Its possible that without a wot test run you may all ready be damaging your motor because you don't know if it is
over propped or suffering some other performance glitch.
Wot is only needed long enough to get settled in at wot rpm.I'm guessing 30 seconds.Your certainly not the only guy out there with an older
boat that needs a test run. If we can get the boat propped correctly it will get on plane quicker with less throttle and stay on plane with less throttle.
I can tell you that your numbers at 3200 are suspicious.
 

Sbrent

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Ok I'm away working right now I will get a wot test run when I get back home I tried wot for about 30 seconds or so. Didn't get an accurate reading on the tach. Thanks for the info!!!
 

Sea Rider

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Now a days 600-1000 rpm are the min and max wot range revs for most outboard engines. Under min side you're lugging the engine, over max you're over revving the engine. Both with bad engine consequences. Go for a wot test you alone with well trimmed combo on flat calm no wind water cond if possible and check max wot rpm achieved. If readings are suspicious, not accurate install a cheap induction Tiny Tach to corroborate actual readings before maximizing prop,

Happy Boating
 
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I have an older Prestancia with twin 4.3 Cobras.. 1100 hours on them 205 hp each...and 2 or three times a season i give her the full throttle test... Light boat, full throttle watch the revs and don't let her overrev.. Adjust the engine trim, leave the trim tabs up and then check the gps speed and rpm. I get about 4600 rpm at 40.4 mph... Her best cruise is at about 29-30 mph and 3500 rpm.. At less than 3200 rpm / 25 mph she wants to come out of plane and will labour to stay in plane... So.. After reading this I realize that your boat is somewhat different however... I am surprised that your boat stays on plane at 20 mph unless the trim tabs are down...and the engine trim is full down.. That being said, I would try the test as I stated above... Make sure the trim tabs are up to not create drag...trim the drive up as required to get her to settle to a nice plane, not bow down.. . But be cautious of going above 4500-4600 rpm especially as you trim up the drive and get a good gps reading for speed.. 4.3's have no issue at 3500 rpm and can do it all day long as long as they are efficiently moving the boat and not being overworked for the speed the boat is moving.. Sorry but I doubt that you will see 2500 and stay on plane without overusing trim tabs and a low engine trim (not efficient and hard on the engine) A Tara is a fairly large boat for one 205hp drive... And I would think that 3200rpm would be the bottom rpm to stay in plane... I would think that 3500 would show close to 30 mph and mid to high 30's at wot... This is only my opinion though...it would be best if you heard from someone who currently owns a boat the same as yours..
 

steelespike

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Yes I agree it is great advice thanks. But your saying best economy through the throttle range, if that's the case than I only have a 1000 rpm range! Is there anyone out there who has about an 4000 pound boat with a 4.3??? What is your throttle range??
WOT rpm range is just that at wot the rpm range is 4400 to 4800. Not saying you need to operate within that range.But your motor needs to be within that range at wot,light load. Obviously as you add weight the rpm will drop If you carry a heavy load very often you may want to prop to that load and then keep an eye on your tach when lightly loaded.
Once you have the best prop for your needs you can operate at whatever throttle rpm is best suited for the conditions.
 

Sbrent

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Ok so I've just had a chance to test my boat I hade a full tank of gas and 700 pounds of passengers with a small bit of wind prb a 10 inch wave! At wot I hit about 4500 rpm and 35 mph!.
 

steelespike

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4500 @ 35 calculates to 29% slip12% is more typical.Often when slip is high it is an inaccurate tach reading high.
Your 22' cuddy may be cause for some of that slip.700 lbs passengers kind of takes you out of a lightly loaded test run.
Is your speed by gps?If the tach is off your motor could be low as about 3500 rpm.
To further complicate things I have the feeling that your speed should be around 40.
 
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Sbrent

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Yes the speed should be accurate I will use another gps and tach with less weight when I get a chance. I am away working again now for another ten days but start my holidays then so I should get the time to get you an more accurate test!!! Thanks for the info!!
 

Sbrent

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Ok soooo ive finally had a chance to try my boat! I wired in another tach with mine, and it said wot was 4700 and mine said 4400 the gps speed was 49.7 kph there was an spare 15 hp outboard installed on the back and it was three people in with fair water conditions I think this is the same conditions I will be running at 90% of the time so I would like to have it proped to run at this condition! Also I find it takes a bit two long to get on plane and with another person or two they have to stand up front to get on plane! Right now I'm not really using the trim tabs as I am not used to them, I have a medium size hydrofoil on the boat! Oh and just a reminder my prop is a 14.5 x19p
 
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Sbrent

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Ok soooo ive finally had a chance to try my boat! I wired in another tach with mine, and it said wot was 4700 and mine said 4400 the gps speed was 49.7 kph there was an spare 15 hp outboard installed on the back and it was three people in with fair water conditions I think this is the same conditions I will be running at 90% of the time so I would like to have it proped to run at this condition! Also I find it takes a bit two long to get on plane and with another person or two they have to stand up front to get on plane! Right now I'm not really using the trim tabs as I am not used to them, I have a medium size hydrofoil on the boat! Oh and just a reminder my prop is a 14.5 x19p
 

Sbrent

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Any idea on what I need to change??
4500 @ 35 calculates to 29% slip12% is more typical.Often when slip is high it is an inaccurate tach reading high.
Your 22' cuddy may be cause for some of that slip.700 lbs passengers kind of takes you out of a lightly loaded test run.
Is your speed by gps?If the tach is off your motor could be low as about 3500 rpm.
To further complicate things I have the feeling that your speed should be around 40.
 

Sbrent

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Can someone please help me!!!!! I'm thinking about a 15x17 prop with a cup on the blades, so you think this will work any better? Also I've done some research and I read that people who own a larger boat will tend to use as big a propeller the lower unit will handle, and than adjust the pitch, is this correct??
 

steelespike

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Your numbers are just plain screwy 4700 says 40% slip,4400 says 36% slip and I'm convinced that wot should produce close to 40 mph.
I do wonder if the boat has absorbed water in the flotation foam.Slow hole shot with a foil on the leg is also a little screwy.
The foil will very likely cost a little top end speed.You certainly don't need a foil and trim tabs.
Yes a 17" pitch will improve hole shot and load ability.But will likely increase rpm at the various speeds.
While runnig at slightly higher rpm it will be straining less And likely will turn more rpm with less throttle.
Are the trim tabs up out of the way?
You may be able to check for water in the foam by raising the bow on the trailer leave out the drain
plug and collect water that drains once the loose water is gone.If there is saturation it may drip for a very long time.
You could also remove excess weight and weigh the boat compare to fry weight.
You'll need to weigh the trailer and boat and then just the trailer.
 

Sbrent

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Ok well the trim tabs are all the way up. As for the water in the hull, I left it draining for 10 days last shift while I was working I usually take the drain plug out and raise up the bow to rid any water! But how long do you think it will take to drain/ fill up with water? We did have a couple days of rain before I tested the boat but the pump took out what was under the motor! So how does the slip calculation work? If I have two much weight, than the %of slip will be high? But if I'm carrying the lowest amount of weight that I run with and the slip is still really high what does that mean? I have another tach that I will hook directly to the engine and take a picture of that and the gps to show you! Thanks a lot for your help!!!



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steelespike

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If it was still dripping after 10 days it is very likely saturated foam.I'm not sure where or how much foam a cuddy has.
Some folks use a hole saw to gain access .to check some samples go just far enough to expose the foam.
In a prop calculator you put in the rpm,gear ratio,pitch and gps speed. this give the slip.Slip is the hardest number to get.
The gear ratio needs to be accurate, speed by gps, pitch on the prop.These numbers are usually accurate. Rpm is always suspect
until compared to gps speed. Slip is the measure of the actual speed the prop goes through the liquid water measured in percent of slip.between theoretical
speed and actual speed. I use 12% for a typical runabout if i need slip for some calculation. As the prop rpm go down slip usually goes up.
Truly heavy or hard to push boats tend to have high slip.Your slip is off the chart.Perhaps a number of causes make it appear you have very high slip.
 
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