Performance/ prop selection for Seaway 24 offshore

TurtleIV

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I got a Seaway 24 offshore this winter and love everything about the boat except that it does not perform up to the published specs. Engine is a Yamaha F150 on the Armstrong bracket. The prop is a 3-blade SS 17M 14.25, and I just had it checked out for problems and the hub professionally replaced.
The boat seems to accelerate well enough, but does not maintain planing speed well, for example turning even a little when running at 24 mph for example will sometimes cause the boat to dramatically slow (like down to 16-17 mph), and the engine to rev like the prop is experiencing cavitation. Also, getting the boat up to fully planing (over 20mph) requires running the engine over 4000 rpm, which is much more than the published data from Seaway, and seems excessive. The engine will exceed 5500 rpm at WOT, but the boat will barely exceed 30mph at that rpm. This is far less than published data. During this testing, the boat is lightly loaded, half or 3/4 fuel tanks, one person aboard. The bottom is clean paint, not the smoothest job out there but no barnacles or growth, I'm in fresh water and keep it clean.

Seaway publishes the following performance numbers for this:
3000 rpm 16.5 mph
3500 rpm 23.5 mph (most efficient cruise speed)
4000 rpm 28 mph
4500 rpm 32.5 mph
5000 rpm 35 mph
5500 rpm 38.5 mph

I've tried emailing and calling Seaway about this and no response. Does anyone here have experience with these boats? Or can anyone recommend the best prop for it, or let me know what else may be causing this? Thanks!

at steve house.jpg
 

Powderpro

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Believing published performance numbers of a boat is like believing a politician is telling the truth. I would not expect a 150hp outboard to push your 24’ boat to 38.5 mph. I think their published performance numbers are unrealistic and are typical of people inflating the speed of their boats. Looking at the size and finish of your boat, I’m a little surprised you are getting 30mph out of only 150hp. Your tall windshield and hard top grab a lot of air, the boat looks well built, My opinion is the boat is a little underpowered if you are wanting/expecting the published performance numbers (38.5 mph top speed).
 

Haddock

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The new owner of Eastern and Seaway boats has joined this forum; I will look for the thread where he introduced himself here somewhere. You can PM to discuss your boat and performance numbers and see what he can recommend.
 

Codcruncher

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Powder pro is right about the power...the speed reduction in a hard over turn is normal...some of it depends on your trim angle...if I'm going to make a hard over turn at high speed I bury the prop during the turn, she will always slow dramatically without touching the throttle,if your making a turn without adjusting trim angle you will get cavitation causing the engine to rev and the boat to slow, then trim back up after the turn for plane...typically the boat should be on plane at cruise when the top of the engine is level, approximately...you will feel the bow go down and speed increase as you adjust trim
 
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tsharac

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From the Seaway brochure, which prop did they use?

Roughly how slight of a turn are you seeing the speed drop from 24 to 16 mph? 5 degree turn? 30 degree turn? What happens when you trim in before you take these turns?
 

TurtleIV

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The boat loses speed in a gentle turn, and does it a lot more if you turn it a bit harder. This is not about what overall angle I am turning, but that the turns are gentle, meaning sweeping on a long radius, using very little steering input, like maybe 10 degrees turn at the wheel. Other boats I've had like my 17' whaler would not do this at all in gentle turns like this, it would easily stay on the plane in gentle turns without throttling up, and no sign of cavitation.
The trim does not seem to have any great effect on this issue or the speed in general, this boat does not appear to be very sensitive to trim. Trimming all the way down gives about the same speed as trimmed level, but the steering is twitchy and the boat does not track well trimmed all the way down. The boat feels a bit unstable in yaw at speeds over about 25 mph regardless of the pitch, but this gets worse when trimmed way down.
I wish I knew what prop Seaway Boats used to generate the performance numbers, they have not contacted me back from my email and phone requests which is why I made this post, I'm trying to get this information. I hope this is a prop selection issue, or something else that I can fix.
I've attached the chart from what I think is the correct Seaway brochure below. I was wrong about the top speed above, but my main concern is the mid-range performance anyway, at 3500 rpm for example.

24 offshore performance.JPG
 

tsharac

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The boat loses speed in a gentle turn, and does it a lot more if you turn it a bit harder. This is not about what overall angle I am turning, but that the turns are gentle, meaning sweeping on a long radius, using very little steering input, like maybe 10 degrees turn at the wheel. Other boats I've had like my 17' whaler would not do this at all in gentle turns like this, it would easily stay on the plane in gentle turns without throttling up, and no sign of cavitation.
The trim does not seem to have any great effect on this issue or the speed in general, this boat does not appear to be very sensitive to trim. Trimming all the way down gives about the same speed as trimmed level, but the steering is twitchy and the boat does not track well trimmed all the way down. The boat feels a bit unstable in yaw at speeds over about 25 mph regardless of the pitch, but this gets worse when trimmed way down.
I wish I knew what prop Seaway Boats used to generate the performance numbers, they have not contacted me back from my email and phone requests which is why I made this post, I'm trying to get this information. I hope this is a prop selection issue, or something else that I can fix.
I've attached the chart from what I think is the correct Seaway brochure below. I was wrong about the top speed above, but my main concern is the mid-range performance anyway, at 3500 rpm for example.

View attachment 108375
These performance numbers are very perplexing.

First thing that sticks out: the hull goes 29.6 mph @ 4000 rpm, but 26.7 mph at 4500 rpm?

Second thing, at 7.5 mph @ 1500 rpm the hull gets 5 mpg, for a range of 330 miles. Then at 30.5 mph @ 5000 rpm the hull gets 5.2 mpg, for a range of only 233 miles? 100 miles less range at a higher fuel economy.

Third, the worst fuel economy at cruising speed of 29.6 mph @ 4000 rpm gets 3.8 mpg and a range that's farther than when the hull gets 5.2 mpg at 30.5 mph @ 5000 rpm.

Fourth, the gph varies in an unusual pattern, peaking at 4000 rpm @ 29.6 mph @ 7.9 gph and then starts to burn fewer gallons per hour at higher engine speeds.

Fifth, normally gas outboards burn approximately 10% of the horsepower rating at WOT in gallons/hour. And at optimum cruise burn 1/3 that gph and cruise at 2/3 top speed in mph. This outboard should be burning ~15 gph at WOT, but somehow is burning only 6.9 gph at WOT.

The inconsistency with the hull speed as a function of the engine speed, the inconsistency of the rated range not matching different fuel economy ratings, and the especially low fuel burn numbers at WOT all lead me to believe there may be some errors in that performance chart that prevent it from giving you an accurate assessment of what your boat can and should be doing.
 

Shark49

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Your boat is running fine. We have a 24HTS 500+ hours 141/2x15x4 stainless 4400 cruise 21 knot, WOT 28 knot. I have a 135 Honda. I have never experienced reduction of speed to that extent in a turn, but usually run no trim in that instance. I have my 4 blade aluminum if you want to try it or call CJ at Twin Hull boats and get a new prop. PM me with questions.
 

Shark49

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Just looked at picture again. That kicker will create lots of drag if you leave it down while running.
Nice boat. Where do you keep it?
 

RobBaker

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I too have been unable to achieve Eastern's Performance numbers for my Sisu 22. Perhaps because of my painted bottom, wind, current or weight. Love my boat regardless it's a great boat.
 

Shark49

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I doubt Eastern/Seaway was able to achieve those numbers. I know when I Sea trialed my 24HTS CJ said that much over 30 knots the stern will get squirrelly since the Hull flattens out at stern, he was right. I can sometimes get 32 knot with the right conditions but mostly 27 ish.
 

El Mar

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WoundUpMarine

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TurtleIV

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I always keep the kicker up, it is fully out of the water when running the boat. Thanks for the answers!
Shark49, do you mind telling me what prop you have on yours? Do you have the Yamaha F150?
I don't care about going fast, but would like if the boat could maintain a more consistent speed in the mid range (20-24 mph) without the boat coming off the plane/ prop slipping or cavitating when I turn.
 

El Mar

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Brooksie

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The boat looks overweight. I'll bet to get anywhere near the advertised speeds and consumption numbers, you need to shed 1000 lbs. Look how deep the transom is immersed right down to the quarter guards.
 

Spence

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My numbers are from my chartplotter with NMEA interface to engine on '13 24 HT Sport (all the 24's are the same hull) with 115 Yamaha with stock 3 blade prop no bracket. At 4 k rpm's I get 18 mph (normal fash cruse), at 3.5-5 k rpms 3.5-4 mpg. WOT 31mph. At 1800 rpm 6-7mph and 6-7 mpg. Note that my '13 hull has no liner and weighs 1000 lbs. less than current models. NB 150 is max hp for the hull
 

Keelboater

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All good info above. One thing to remember is that quality control has turned to crap across the board. So it would not surprise me if that prop might have a compromised bushing right off the shelf. Not saying it is or isn't, just putting the concept of good parts and bad parts on the table rather than new parts and old parts. It's the new way of the world. I could fill the back of my truck with brand new defective parts that I've purchased. It's become so bad that I can honestly say if I were in my 20's again, I would never tackle any do it yourself projects. :mad: That's a nice boat you have there. Best of luck with it.
 
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