Prop slip and boat speed questions...

[vissertrades]

Hi,<br /><br />Just got a GPS and went out testing different props. Link here has some background info (15.5' Smokercraft aluminum with '95 Johnson 50 VRO:<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=034497 <br /><br />Anyway, here are the results (me and 50lb son + 12 gal gas):<br /><br />1. Prop - OMC 11-1/2x19 Aluminum (new)<br />5600 rpm 33.5 mph.<br />2. Prop - OMC 11-3/4x17 Aluminum <br />5800 rpm 32 mph.<br />3. Prop - OMC 11-3/4x17 SS (looks cupped less than aluminum)<br />6000+ rpm 31 mph.<br />4. Prop - ?? 12x15 SS (heavy cup)<br />6000+++ rpm 30 mph. (almost jumped out of the water)<br /><br />I do have Smart Tabs installed at a 25 degree angle and actuator on weakest setting. I do have a OMC 11-1/4x19 SS yet to try. AV plate is currently 1/2" above hull and 19" prop vents easily in turns or from a non-straight take-off (breaks loose)...<br /><br />Here's my questions.<br /><br />Should this boat go faster? <br /><br />Is slip reasonable for a semi-V aluminum (18-20%)?<br /><br />What set-up should I chose?<br /><br />Should I put my beer down and slowly back away, ?<br /><br />Thanks!

 

[JB]

Re: Prop slip and boat speed questions...

Hi, Piet.<br /><br />Looks like the 19" is the best of the props you tested. Partially because that gave you the best speed, but mostly because your WOT rpm was correct for the engine.<br /><br />33mph is good for a 50hp engine on your boat.<br /><br />Good luck.

 

[Lark40]

Re: Prop slip and boat speed questions...

Piet,<br /><br />I just did GPS/prop testing with my antique boat, which is proportionate in weight and power to yours.<br /><br />Similar results - best WOT of 32 mph when at 5600 rpm. Lot of prop slip when trying less pitched props with higher rpm. I think you would need a larger engine to get any more speed.

 

[Jdeagro]

Re: Prop slip and boat speed questions...

Piet;<br /><br />What is the Smart Tabs Model Number and actuator pressure rating. In your previous posting you indicated that the 11.5 x 19" prop seemed slower and more sluggish out of the hole. Here you indicate that it is the fastes at WOT.<br /><br />Do you know what the top speed of the boat was prior to the Smart Tabs installation? <br /><br />If not simply disconnect the actuator at the transom bracket (top), pull it up and tie it off so the plates are not functional. Now run the boat with this 11.5x 19 prop and see what the top end is. Then reconnect the actuator and make the same run in the same direction. <br /><br />Sound as though 33.5 mph is a bit above the average for this boat. You should also keep in mind that with this HP a small amount of extra weight will impact the top speed.

 

[vissertrades]

Re: Prop slip and boat speed questions...

John,<br /><br />Tabs are the ST980-40 (9"x8" 40lb actuators). The GPS numbers above were with tabs at 25 degrees and actuators in the center of adjustment. It "felt" like the bow was plowing. I had limited time so I didn't GPS after adjustment, I will ASAP when I test the 11-1/4x19 prop. The 19" prop from above is slower (sluggish) except for top end when compaired to the others. Also, It seems to ventilate easier than the others, not sure why. With your tabs I'm able to leave the motor trimmed out and still pop on plane within reason. The boat is more stable and seems to need less steering correction at low speeds (maybe due to trim out position).

 

[Jdeagro]

Re: Prop slip and boat speed questions...

As you suspect it may be necessary to reduce the pressure ( move the actuator closer to the plate at the Plate bracket only) Do not go all the way to the bottom as you do not want to loose ride control. Do not change the angle of 25 degrees ( at the top transom bracket).<br /><br />If you trim the motor all the way down, as you have done in the past without the Smart Tabs, it should come up on plane fairly easy and quick. You can then trim the motor when on plane to the best pro angle.<br /><br />The ventilation on take off may be caused by the starting trim angle, you may need to keep it down. It could also be the design of the prop blades. I would suggest that you start off with the motor all the way down, and instead of pushing the throttle hard reduce the initial acceleration rate a little, then once you are moving increase the acceleration.<br /><br />My guess however it is the prop. Try disconnecting the actators using the same prop and see what happens. <br /><br />The fact that you are running faster at the top end but seems sluggish at intermediate speeds, leads me to believe that you will need to reduce the pressure just a bit. Keep in mind that the boat will ride smoother and therefor may not feel as fast.<br /><br />The low speed wander went away because the plates are stabilizing the boat, and resisting the side to side movement.

 

[vissertrades]

Re: Prop slip and boat speed questions...

Went out to try the 11-1/4x19 OMC SST prop. It was faster than the 11-1/2x19 OMC aluminum by about 2 mph. Also WOT rpm's went from 5500 to 5800. I noticed the same rpm increase with the 11-3/4x17 SST -vs- aluminum as well, but no increase in speed. I thought the stainless props reduced rpm??? The exhaust opening is larger on the SST props and cupping "seems" less than on the aluminum. I liked the overall feel of the 19" SST prop. I didn't get to play with/without the Smart Tabs yet but they were setup in the weakest position and boat pops right on plane. I may need the 30 lb actuaters and center them in the middle of the adjustment slot. Last time out weather was bad! 25-30 mph winds and we got soaked! I'm running out of good weather so I'm not sure when I can get out next...

 

[Jdeagro]

Re: Prop slip and boat speed questions...

Piet;<br /><br />The difficulty in nailing down performance figure when testing in the water ( which is the only way you can) is that the conditions change constantly in the same day let alone a week or two later. You have your hands full! Anyway, it now appears that you are able to get 35 MPH+, which sounds pretty good for a rough day. Once you deside on the best prop set up, this is the way to determine the best pressure setting for the Smart Tabs. Pull the tabs up (manualy) and tie them off so that they are not functional. run the boat WOT from point "A" to Point "B" and record the RPMs and Speed.<br /><br />Go back to Point "A" and run the same test except with the tabs functional. The boat should run about 2MPH+ faster, if not then reduce the pressure.<br /><br />Keep in mind that you seldom run the boat at WOT, and the pressure when cruising is what gives you the handling, and ride control. So reducing the pressure and increasiing the pressure will alter the se other characteristics. Under no circumstances do you want to run with too much pressure and force the boat to run bow down. However minor adjsutment will allow you to tune the boats performance and handling to your specific needs.<br /><br />Call if you want 800-233-0194.

 

[vissertrades]

Re: Prop slip and boat speed questions...

John,<br /><br />This time I was only able to get 32.5 mph with the SST prop and 30/31 out of the aluminum. Conditions were less than ideal and I only had a small strip of "calm enough" water to play on. I don't think the lower speed was tab adjustment related as I didn't have a chance to trim out like I did before (maybe it was?). I had planned on tuning the tabs but my 9 year old soaked boy wasn't having as much fun as I. I will try to get out again and run some tests ASAP. My only concern is that I'm allready adjusted on the weakest setting and the bow still feels planted, keep in mind this is a console boat with four seats, one rear, drivers and two bow seats (one in casting platform and other just rearward). 99% of the time the bow is loaded via passangers and junk. Can I buy a 30 lb set of actuators if needed because It may be next spring before I get this dialed in?<br /><br />Thanks!

 

[Jdeagro]

Re: Prop slip and boat speed questions...

I understand, and yes we will sell you 30 lb actuators if required. Some how 9 year old's don't have the same technical interests as their Dads. Keep him dry, warm, and loved.